PC Master Race

I’m getting a red motherboard light for CPU

Is it flashing, or just solid? Some Motherboards use LEDs to provide a trouble code, this will be a specific sequence of flashes which detail what the problem is. Look at your motherboard manual and see if it has anything about the light and what it might be telling you.

Also, is the system beeping at all? System beeps are a really old method for giving trouble codes. If it is beeping at you, again refer to your manual to figure out what the beeps are telling you.

If you have someone around who can loan you a PSU, that would be an easy test to run. Even if the PSU is underpowered for your GPU, the system should at least POST and let you get into the BIOS.

Once you get it booting, the failure under load could be a couple of issues. It could obviously be the PSU and that is the root of everything. But, there are several things which might be worth checking. Specifically, I'd check the thermal paste on the CPU and GPU. If you've done any work on either recently, it's possible that you got a bad application. That stuff also wears out. My wife's system had a similar crash under load which turned out to be the GPU overheating. That was caused by old thermal paste not working well. Replaced the paste and she got another year out of that card.