So I was testing out a new harddrive, and completely removed my normal one. Once I established it was good, I threw my original one back in too, and while I was at it I moved my PC to a top of desk position vs under, sprayed everything with air, and unseated, and reseated everything I could access with just the side open.

Now, it's dead. The USB ports are still powered. When I plug in the power supply the light on the power button turns on for about 3 seconds, then flicks off. If I push the power button while it's on the fan starts to spin, but it dies with the power button LED.

What on earth could I have done?? I didn't touch a thing while it was plugged in.

TLDR; swapped an HDD and moved tower location, now PC won't start. Tried different cords and power sources.