I'm also going to go with PSU dieded.
I experienced something similar.
It worked one day, died sometime in the middle of sleep mode and wouldnt wake. no post or nothing. MOBO lights were still on and pulsating though.
I left it alone for a day (turned off PSU), and it started right up again the next day.
Soon after same thing happened but nothing would make it wake anymore and when it finally did it didnt stay awake for very long. I put in a new PSU and havent had any issues since. Lucky for me none of the times it died was when i was using the PC, so it either died in sleep mode, or it died just idling.
I've had that PSU since building a Phenom II system way back when, so it most likely outlived its useful service life.
I hope so. I don’t have a backup PSU to try. I did buy the tester Bill linked to see if that also points to the PSU. The thing is supposed to be on warranty. If the tester says it’s bad I am going to see if I can get a replacement. But the headache may not be worth the $120 a new one costs. Let’s see what happens when the tester arrives.