Computer suddenly turned off and it won't turn back on. Please help.

[Drakiam]

Hello guys. I need help. My computer suddenly turned off while I was watching a video and it won't turn back on. I have a Sabertooth Z77 motherboard and the green power LED is still on. When I tried starting it by shorting the PWR pins, it seemed to start for half a second and then it died. The computer had been working fine for months and I find this really weird. What should I do? The PSU doesn't seem to make any noise when I try to turn it on but it does make a clicky noise when it's switched off. I'm not really experienced with computers. This was my first build so don't expect me to know all the complicated stuff. I had actually had 0 problems with the computer, I could play games, I could do anything. I was rather surprised when it suddenly turned off today. I wasn't doing anything intense and I had been able to play a couple of games before.
UPDATE:
PSU is a Corsair Hx 750

 

[Drakiam]

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[tasha333]

Hello guys. I need help. My computer suddenly turned off while I was watching a video and it won't turn back on. I have a Sabertooth Z77 motherboard and the green power LED is still on. When I tried starting it by shorting the PWR pins, it seemed to start for half a second and then it died. The computer had been working fine for months and I find this really weird. What should I do? The PSU doesn't seem to make any noise when I try to turn it on but it does make a clicky noise when it's switched off. I'm not really experienced with computers. This was my first build so don't expect me to know all the complicated stuff. I had actually had 0 problems with the computer, I could play games, I could do anything. I was rather surprised when it suddenly turned off today. I wasn't doing anything intense and I had been able to play a couple of games before.
UPDATE:
PSU is a Corsair Hx 750

Just today I had a similar experience, computer turned itself off while playing a game and wouldn't switch back on. I assumed it was the psu however after trying a spare it still would not power on. I stripped the computer to the essentials,(MB, CPU, 1 RAM and graphics card) tried both psus but still nothing. Almost gave up but
Removed the graphics card tried again and lo and behold the motherboard powered up. Now looking for a new graphics card.