|Compressed air has a high esd damage along with a possibility that you blew trash, solder or other into some connection.|
Remove ac plug and press power button a few times then.I'd take the system and shake it with the cases open and maybe consider using a esd safe vacuum to try to remove any debris that might be causing it. !!! THE NEXT STEP ASSUMES A MODERN PS!!! It may be that your power supply switch has line voltage on it and can kill you. Test with Voltmeter before you attempt this. Then check all connectors and retry. If fails then do PS diag with paper clip and one hard drive connected.
Playing to the angels
Les Paul (1915-2009)