Power down and restart

I have a 3 year old pc that has begun spontaneously powering off, and then restarting within 30 seconds. I have taken the following actions thus far:

cleaned fans and components, McAfee virus scan, replaced power supply.

After replacing the power supply, the system no longer powers back up, I have to press the power button on the front panel….throughout all of this that button has remained lit when the pc powers down, so now I am thinking that something is sending it into hibernation??