So, basically every time I open my computer I am afraid I will somehow permanently damage it, though I have had some scares I have managed to figure out the solution eventually, but I am completely lost right now.

I wasn't even doing anything major, just trying to see if I could fit a fan into my case as I found the HDD was running rather hot. It was far too big so I just put the thing back together and, to my horror, it would not turn back on. There appears, only sometimes, a brief flash from one of the lights on the case (it's for SD cards or something), and other than that, absolutely nothing. I am sure all the cables are connected (at least 99% sure).The only thing I unplugged was the 20/4 pin that connects the PSU to the mobo, and it is plugged in, I made sure many times. I only lightly nudged the RAM sticks a bit, not with any significant force but could they have dislodged or something?

My greatest fear is ESD, and because of this I maintain almost constant contact with the PC's metal case. Despite this precaution, is it likely that ESD has damaged a component? Perhaps when trying to plug a cord back in, my hair came too close to a component. I could send pictures or something of the inside of the case, but I really don't believe any of the cords to be unplugged or anything like that.

Hi Devin and welcome.

I am sorry you are having so many problem, especially when it appears you have taken the necessary precautions - especially concerning ESD. If you are in near constant contact with the bare metal of the case, that will discharge any static in your body, and prevent more static (as you squirm around in your clothes) from building to damaging potentials. So I don't believe this is an ESD issue.

Was the power supply unplugged from the wall during all this? Note when a PSU is plugged in (and if equipped, the master power switch on back is set to ON) the PSU delivers +5Vsb standby voltages to several parts of the motherboard. If you connect and disconnect the motherboard's power connector with this +5Vsb present, that might have some adverse affects.

Why did you unplug the main 20/4 connector? You would not need to do that to add a fan.

Note too, newer motherboards require two connections to the PSU, the 20/4 pin connector as well as a 6-pin Auxiliary connector. So make sure that one is connected firmly too.

If 100% sure (99% is not good enough), then I would swap in a known good PSU and see what happens. Note there are many other cable connections to other devices, including the case's front panel, that you need to ensure were not knocked loose.

Finally, while I understand the hesitation to dig around inside, it is a user responsibility to keep the interior free of heat trapping dust. So I generally recommend opening up the side panel monthly to inspect, then clean as necessary. But as noted, before reaching in, take the necessary ESD precautions, and make sure the PSU is unplugged from the wall too.

Thanks for the response. The PSU was unplugged, during all this. As for why I unplugged the main connector, don't laugh, but I thought the fan might be able to be installed right next to the HDD and the way my motherboard is set up, the massive cord was directly in the way. When I return home I will check to make sure connections to the case's front panel were not knocked loose. My mother board has a 4 pin auxiliary connector that IS assuredly plugged in.
I don't really believe the PSU to be the issue, as the unit appears to have power. When an Ethernet cord is plugged into the back, there is a green light, and like I mentioned that brief blue LED lights up when I first press the power button.

Well, note the PSU must supply +12, +5V and +3.3V and it must supply +5Vsb standby voltage when the computer is turned off but still plugged in. So one or the other may not be working. Here is my canned text on testing PSUs:

To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

Note the required voltage tolerance ranges:

Swapping in a known good supply is a tried and true method of troubleshooting used for years, even by pros. If you have access to a suitably sized, spare power supply, carefully remove the suspect supply and replace it with the known good one, and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

Note the ATX Form Factor standard does not "require" specific color coding for power supply connector wiring. It has recommendations but manufacturers often do not follow them. Sadly, many testing guides or tutorials will refer to wire color only and that can lead to improper testing.

The voltages can be checked in the BIOS Setup Menus of most motherboards but they do not reveal ripple or other anomalies either. And of course, booting into the BIOS Setup Menu requires a working PSU.

As always, before working in the interior of the computer case, take necessary ESD precautions to ensure static buildup in your body does not discharge through and destroy any sensitive devices. Unplug from the wall and touch bare metal of the case before reaching in. And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user-serviceable parts inside a power supply. If you do not have a tester or a suitable spare to swap in, take the PSU to a qualified technician for testing.

For more information on testing PSUs, see this excellent article by Gabriel Torres, Why 99% of Power Supply Reviews are Wrong.