You said "It quits immediately after a hard reboot." This implies it was running immediately prior to you doing this...?
You need to start from square one. Disconnect power cables from all drives. Disconnect all ribbon (data) cables from the motherboard connectors that lead up to your various drives. Remove all plug-in cards from motherboard slots. Disconnect mouse and keyboard, and anything else that's plugged into the rear motherboard sockets. Remove RAM. Carefully identify and remove the front panel connectors (reset, power switch etc) from the motherboard headers.
Check that the CPU fan rotates freely. Now try powering up the motherboard. You will need to momentarily short the power switch connectors since you've removed the front panel switch. Does the CPU fan rotate? If it just "jumps" as before, check the PSU fan. Is that turning?
If not, it's pointing towards a PSU failure. Has the motherboard been disturbed at all? Any likelihood of a screw having got underneath and shorting out part of the circuitry? The only way to check thoroughly is to remove the motherboard from the case. Now you have the motherboard out, lay it on a piece of cardboard or other insulating material and try powering it up again. This will at least prove it's not being shorted out.
ROGER - G0AOZ.