My desktop won't power on

My desktop is an HP A1620n. When I press the power button in the front it does nothing. No fans or sound. I plugged the pc into a working outlet. There is no on/off switch for the psu. There is a green light on the psu that is solid green when plugged in. Any problems I found online were associated with a blinking green light or no green light. I tried troubleshooting the internal hardware and power supply for things like the cd-rom and no luck in powering on. The light is always solid green when plugged in. The strange this is after 4 hours I put everything back together except the outer case and it did not start up. But while I was putting the case on it started on it's own. The pc ran fine all day. The pc powered on and off fine. I shut it down for the night and in the morning it wouldn't start again. I was reading about maybe the motherboard is shorting out on the case but I don't know if that is a real thing. Any help would be appreciated. Also there was no power outage or lightning storms that happened.
Thanks, Matt.
 
sounds like there is a possible ground fault or something is triggering chassis intrusion (i.e. the side panel is off).
 
Everything I read about chassis intrusion says u will get a message about chassis intrusion when u turn on your pc. Nothing about keeping u from turning on your pc.
 
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