I was just wondering if there could be anything other than a broken power supply or motherboard that would prevent a computer from turning on, and if so what are they?
PSU not plugged in electric outlet. ^_^
Broken AVR (if PSU is connected there).
Faulty case power button and wire not properly connected to motherboard.
Loose wiring.
No power at wall socket
Faulty power cable
Faulty power switch on the psu
Faulty psu
Faulty motherboard
Powersw wire not connected to the mobo from front panel or connected in wrong orientation
Case power button not operational
Atx 24pin cable not connected to the motherboard
Then it could be the PSU or motherboard. Can you test that the PSU itself works by jump starting it? If that works, then try plugging it into the motherboard and this time, leave out the GPU.