Water got into my desktop while it was off, including the PSU. What should I do?

Keep it unplugged and don't rush a test boot. Being switched off helps, but a computer can still have standby power when plugged in.

Remove the accessible components so you can check for trapped water around connectors, fans and heatsinks. Blot visible water carefully without rubbing exposed contacts, then leave the parts somewhere dry with unheated airflow for several days. A few days isn't a guarantee that hidden pockets are dry, so inspect them before putting anything back together.

I'd replace the PSU if water actually entered it. It might still work after drying, but a failure could put other parts at risk. Do not open the PSU case, even when it's unplugged. Its internal components can retain a dangerous charge.

If you want to explore reusing it, take it to a qualified repair shop for assessment. A basic PSU tester can check some outputs, but a pass doesn't prove the unit is safe or reliable after water damage.

Drying isn't the whole story. Pipe water can leave residue, and corrosion can cause trouble later even if the machine starts up. Check the boards and connectors for deposits or discoloration once they're dry.

If there's visible residue, a repair shop can clean and inspect the affected parts. I wouldn't pour alcohol into the PSU or start rinsing assembled components when you can't see where the liquid will end up.

Don't assume every part is ruined, but don't assume they're all fine either. Once everything has been inspected and dried, test the system with a known good PSU and as few connected components as practical. Add the remaining parts back gradually and watch for problems.

If it does start, back up any data you care about promptly. A successful first boot doesn't rule out delayed corrosion or failure.