Desktop suddenly turned off and won't turn on again

Jump it with the paperclip, then test your cables to make sure they're getting the correct voltage.

It may also be that the PSU isn't getting a good PWR_OK signal. You'll need a tester to determine that as the timing is in the millisecond range.

You can also try using another known good PSU to try it to see if it's the PSU or the motherboard causing issues.