Dell PSU? Some Dells had a proprietary pinout on the ATX power plug.

Except for not being a Dell, we had same symptoms at work yesterday. PSU went out, no start at all with a new one. Unplugged things until it started, turned out PSU failure had also spiked the hard drive. Fortunately it was only a shorted protection diode on the drive's control PCB, removed that and it's operating.