My PC is just 1 year old, no major problems since I bought it.
All the sudden last night it would just not power on at all, as if it was disconnected from the mains.
Checcked all usual things of course, plug, fuses, switches etc, all ok.
Opened case,a green led on mobo (ASUS) was on, switched PSU on/off several times through back switch, this led responded to it accordingly. Also fans briefly turned for a couple of seconds when PSU power switch was turned off.
Removed front on/off pushbutton, pressed it several times, no joy.
I left the room to count to 100 (always a good method to keep calm), when I came back the PC was on!
logged in, all working 100% normal, no error messages whatsoever, just fine.
Turned off normally through Windows and tried to switch it on again, same problem occurred.
done usual manipulation with turning PSU on/off several times and also on/off pushbutton.
After a few minutes PC turned on and worked normally as usual.
Someone told me it could be a faulty mobo, hope not, why would PC run smoothly when on otherwise?
Could it be PSU or on/off microswitch?
I have a spare PSU that I could try out, any other suggestion on top of this?
PS, please don't suggest using warranty, PC is a PANRIX, I was one of the lucky last to receive theirs before they went bust!

It could be the PSU on it's way out, or it could be the motherboard as someone suggested...

you can test to see if it is the switch by shorting the 2 wires on the switch (or metal connections)

Check that the wires do actually connect to the switch and are correctly seated in their sockets (at the mobo)

It may sound stupid but have you dis-connected and re-connected the 20/24 pin ATX (power) connector on your mobo??

Also, when you attempt to turn your PC on, does the PSU fan spin?? If not, un-plug it and put a wire between the green wire and black wire (for most newer ATX PSU's) this should turn the PSU on and your PSU fan and hard drive should come on please note: this is completely safe as I have tried it myself. Also, on older PSU's (or other non-standard ones) there is no green wire so you can't get mistaken

Thanks Mike,

No I haven't tried disconnecting and reconnecting 24 pin connector, will do it.

When I try to power up the PSU fan doesn't spin at all, as I say in the post, it behaves like if there was no power from the mains.
This is not true however as the green led on mobo is on.
Ironically all fans have a short burst of spin whe I turn off the PSU switch located at the rear of the PC.
This is what makes me think of a PSU related problem.

Sorry but I think I haven't really understood this last bit with the PSU fan.

What do you mean by disconnecting the PSU fan? This is internal, do I need to open up the PSU?
Also does this need doing by leaving it connected to everything else? Quite difficult to get inside the PSU without removing it from the case.

Thanks

OK I have finally sorted it out.
Bridged green and black, PSU fan turns on but still no power up after reconnecting PSU properly.
After about 4 hrs PC sudently came alive, now working fine as usual.
Problem is: will it power up again tomorrow after I switch off?

bridging the green and black just tested your psu....

if that worked, then your problem i s either the switch wiring/mobo

Will give it a try at changing PSU first anyway, may not be in cause but has also become noisy.
There is a clearly audible "whistle" type noise, usually typical of leaking electrolythic capacitors.
Switch wiring tested and ok.

you could look on your mobo and inside your PSU for bulging/leaking capacitors, be careful when working inside your PC and power supply and i would not reccomend replacing capacitors in the PSU or on the mobo

Changed PSU and upgraded from 300 to 55oW.
All running smooth now, the fault was with the PSU (luckily for me!)
Thanks a lot for your help, I have learned another couple of things! ;-)

glad you got it working thanks for the update