Question PC enters a 'sleep' mode after a few hours of being on, doesn't turn back on.

Aug 22, 2019
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After being used for some period of time, in the range of a few hours, my PC shuts down and enters some sort of sleep or power savings mode. The power LED flashes on and off, but no input from mouse, keyboard, or power button will turn the pc back on. The only way to get it to stop and turn back on is to unplug the power cable completely. I am currently running windows 10, but have tried ubuntu and still had the same issues, so I assume it's a hardware problem.

I have tried updating drivers, reinstalling the OS, checking cables, moving GPUs to different PCI slots, but haven't made any progress.
The power supply and hard drive are new, the 2 GPUs were used by someone else before I got them. All other parts have been used by me before and worked fine.
Any ideas on how to fix this, or what the issue may be? Thanks in advance.

PC Specs:
2x RX 580 4gb GPUs (used, not by me)
MSI 970 Gaming Motherboard
Corsair RM 1000W Gold (new)
AMD FX-8350 CPU
8gb Kingston HyperX RAM (2x4gb)
Western Digital 250gb Hard Drive (new)