Mac Pro no signal anymore

I was watching a YouTube video when all of a sudden the computer shut down. Then I turned it back on and everything seemed normal, but suddenly the image disappeared: a black screen. Since then I can't get an image. I thought it was the graphics card, so I got another identical one, but that didn't help.
I ran the tests.

OVTMP CPU A -> ok
OVTMP CPU B -> ok
No RAM diode problem...

When I press the diagnostic button, the 5V diode lights up when the computer is switched off. When I press it again and start the computer, the following LEDs light up:

PSU
5v
EFI DONE
GPU OK

But the SYS.PG LED doesn't!

Do you have any idea what's wrong? Could it be the power supply?

I plug my iPhone in usb and it’s not charging , keyboard no light. I heard the hdd start and all the fans are ok.

Sincerely

Mac Pro 5.1 - 2* 2,66ghz - gtx 680 2gb mac Edition and asus flashed gtx 680 (I bought from eBay this week)