My brother's computer suddenly won't boot today. He has it on for long periods of time everyday.

When I press the power button the fans just spin once and everything stops.. the light on the motherboard is still on..

does this mean something fried? Doesn't seem like the power supply because the motherboard still has the green light on..

anyone help? thanks

He has it on for long periods of time everyday.

That's no problem, as long as the system is kept clean and adequately cooled.

Doesn't seem like the power supply because the motherboard still has the green light on..

My first thought (and hope) is the power supply. The green motherboard LED means the power supply is supplying a low wattage +5Vsb standby voltage and is a warning to unplug from the wall before pulling RAM or cards. It does not mean the high-wattage 3.3VDC, +5VDC or +12VDC power supply "rails" are working properly. I would test the PSU first off - or swap in a known good one.

Do understand this could be any number of things, to include bad RAM, CPU, motherboard - and PSU. That makes it hard to troubleshoot and starting at the beginning necessary. That starts with power.

unfortunately I don't have another power supply to test it out.. is there any other way to check if it's the power supply?

I use a FrozenCPU Ultimate PSU Tester for power supply (PSU) testing when I am away from home. It is not as good as a qualified technician testing the PSU under a "true" (realistic) load with an oscilloscope or power analyzer, but close. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you can better detect a "failing" PSU, or one barely within specified 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, not a true load.

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 then requires poking (with some considerable force) two, hard and sharp, highly conductive, meter probes into the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it.