PC suddenly stopped working after an outlet tripped the main breaker, any idea what happened?

Hey, I’m trying to help a friend figure this out because I’m not really sure what could have happened.
Yesterday, while he wasn’t home for the weekend, someone unplugged the power cable from the wall. The cable was an extension cord, and his PC, monitor, and the rest of his computer setup were plugged into it.
When he got back, he plugged everything back in and tried to turn on the PC. The USB keyboard wasn’t being detected, and the PC showed a message saying there wasn’t enough power.
He unplugged everything and plugged it back into the same outlet, but that immediately tripped the main breaker and the whole house lost power.
He turned the breaker back on and tried the same outlet again, and it tripped the breaker again.
He then moved everything to a different outlet. The lamps work normally, and the monitor turns on, but the PC itself won’t turn on at all.
The setup had been working normally for about 4 years before this.
What could have happened here? Is it possible the PSU died, or could the extension cord/outlet have caused something else? And is there anything safe he can check before taking the PC to a repair shop?