|Disconnect from domestic power then hold the power button in for about 30 seconds to see if that unsticks it.|
Otherwise I would be inclined to check all internal connections for tightness first (remove and re-insert connectors). You could also clean all edge connectors with a pencil eraser then re-insert cards a few times to clear any oxide off sockets. Include RAM as part of this.
If you manage to get it to run then download the free version of Belarc Advisor, which gives temperatures as well as a lot of other information:
As you have a spare PSU, if none of the above helps then if you swap it over it will prove/disprove if that is the culprit. It's not easy to be certain from afar.
Always pop back and let us know the outcome - thanks
message edited by Derek