No display from PC with graphics card or onboard graphics

After starting up the witcher 3, and watching the majority of the opening sequence, my PC suddenly shut down. It then started back up automatically but nothing was displayed on the monitor.
After fiddling around, reseating RAM modules, exchanging all of my cables and testing different monitors I had no luck revolving the issue.
I then attempted to boot up whilst running of the Intel integrated graphics of my i7 4770K. No luck.
Swapped out my MSI GTX 770 with my trusty old 9800GT from way back when and behold! Everything works just fine.
I tested the GTX 770 on an older system, using the same PSU (unfortualtly its the only one I have) and had the same result. No output to the monitor.

I'm rather confused now. What exactly went wrong? And what's faulty?
Can the CPU be functioning as normal, but unable to run a display?
Is my MOBO at fault for not being able to display off of the CPU?
Perhaps my PSU is malfunctioning and unable to deliver enough power for a GTX 770 or the onboard graphics?

I'd appreciate any help

PSU info?

Yes What PSU do you have exactly?

The PSU is a Seasonic G-series 550w
I've had it for over a year now, running the same components without a problem

does it happen with STRICTLY witcher 3 or with all games you play?

I experienced a few unexpected shut downs playing Crysis 3 (though not immediately, it was a recent issue), but all other games seemed to run just fine. I even started a new play through of Fallout 3 which at one point I had been playing for over 12 hours straight with no issues.
I thought maybe there was a power draw issue, or perhaps a VRAM issue, as it only seems to happen with graphicaly intence games.
However, as stated previously, I can no longer use my GTX 770 at all. This issue has never arisen before.

I believe it may be your PSU. The 770 can draw up to 42 amps on the 12v rail. your PSU you linked only provides 37 amps and over time that can degrade. That explains why it only happens on intensive games like Witcher and Crysis

Edit: The 9800GT only requires 26 amps, which also leads me to suspect power supply.

If that were the case, then why is it that I can no longer boot with the GTX 770, or even with onboard Intel HD graphics?
I can understand shutting down during power intence applications, but surely booting up isn't that intence?

If you are booting with the 770 installed the on board graphics is disabled automatically in the bios, some bios allow you to enable it with a video card installed some require you to remove the video card from the pcie slot.

The other possibility is that the video card it may also be damaged. Your 770 should still be under manufacturer warranty most of them provide 3 years. I first see if you can find a similar card to test with, one that draws at least the same amount of power, and then possibly RMA the 770. I would also attempt to test the 770 in a friends system if you can.

Try clearing cmos.

If that fails try getting the board to post outside the case with only 1 dimm (ram module), cpu, stock heatsink installed.

If that fails try another dimm/slot. If all dimms/slots fail to post then try another psu.

If another working psu fails its probably the board.

Even with no graphics card installed I am still unable to boot with onboard graphics. Which is why I'm hesitant to assume the graphics card is at fault.
Not to mention that my card is no longer under warranty, as I have installed an aftermarket cooler (the card was still functional after the instalation).

Unfortunately I do not have any similar cards at my disposal. I do not have many friends which are PC gamers, and thoes who are have... Let's say sub-par machines. Stock case PSU's and the likes. So unfortunately I'm unable to test components individually

get a power supply tester they are like 20$ on newegg, at least then you know the unit is not supplying unstable power, which might cause the onboard not to boot. Are you able to get the bios screen at all with/without the GPU installed?

I can only boot with the 9800GT installed

When you used the Power supply in the other computer did it boot? Or did you only try it with the graphics card?

Both computers boot using the GT9800, and the same PSU (as it is the only one I have atm).
The only time I am unable to boot at all, is when I have the GTX 770 installed (in either PC) OR the Intel HD graphics of my i7 4770k.

I'm using the same PCI-e slot for both cards when testing, so that's not the issue

The CPU functions just fine when running off the 9800GT, so I don't understand why I am unable to get a display when using the Intel HD graphics

The alternate PC I'm testing on is an old LGA 775 system. It does not have any onboard graphics on with the CPU or MOBO (as was the case with some MOBO's at the time)

Have you tried to clear the CMOS/bios with the reset pins on the motherboard?

Take the video card out, hook up the onboard video, follow the directions in the manual when using the reset button/pins.

I have tried to reset CMOS. No change

It's possible there is something wrong with the motherboard, but it is hard to test without other similar components, I don't have a on cpu onboard but in my server the onboard video is dead. 775 socket.

What you need to do is some how find parts to swap or you'll have to take it to a shop. As to why the onboard intel will not boot, I have no clue, unless something is wrong with power or the motherboard or the cpu's video

But surely if it were the MOBO then I would be unable to boot with the 9800GT in the same PCI-e slot?