In my case I discovered the dog had run across the power strip and had turned it off. It was my son who discovered the strip was off when he checked because his game was not powered up.
What would cause this computer to crash and not turn on for two days and then just now turn on?
Power supply, and or Heat sync.
Your power supply might be faulty . . . Usually a PC will shut itself off if the Heat sync/Fan are not working . . . Most times after about a few hours you can try again, but if the BIOS doesn't see the fan or Heat sync as "Working" on "Post", it will not start.
Poltergeist.