I have recently been experiencing issues with my GPU. A while ago, it stopped detecting my primary video card, so now I'm using my onboard. I had a power surge earlier today while the computer was on. It turned off because of this, so I booted it back up only to discover that it was detecting my video card again. There was another power surge later in the day, and I rebooted to find out that it was no longer detecting my main video card.
The initial problem with the GPU was that it wouldn't send a signal to my monitor, but the fans on it would power on.
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Sounds like a power supply issue, if your PSU is damaged (from power surges) it could still power on the fans of the gpu and even work, but if it gets under any load it will fail. [spoiler]prolly the wrong forums to ask for help about PC issues[/spoiler]