Not enough information to even diagnose that its the PSU. Maybe the CPU overheated due to paste fail. We don't know how long he waited until he tried again. We don't know if the plug to the case was nudged.....we don't know a lot....
To rule out anything but the PSU, hook the PSU up to ONE THING. Does it power it up? If no, then try another thing. If no, then I'd say PSU is shot. (assuming the breaker to the outlet is fine).
If it does power that one thing then you either:
1) don't have enough PSU to power the system.
2) have an overheating something or another, probably a cpu.
3) have a faulty connection between the PSU and the case power on button.