I installed a Pi outdoors, and had (ahem) not quite finished the final touches on weatherproofing it, when we had a lot of rain for a few hours, while I was out.
It was in a Tupperware box with a few vents / cable holes, and the whole thing covered loosely in a supermarket plastic bag. It was plugged into a mains extension also loosely covered with plastic bag (judgement not required, folks, it won't help). Anyway I got home (I remember the red light was still on) and unplugged it and took it out of the box. It actually felt dry, so I thought I had been lucky. But the next day I tried it and it powered on (red light on) but would not boot (no green light). I tried re-flashing the SD from a backup, then from a stock Raspian, then I bought a new SD and tried that. Still, the red light goes on and stays constant but the green light does not come on at all.
I cannot see any rust or water damage. I wonder if water getting in the mains socket could have blown something - but I suspect the red light would be off in that case. I can't see any burns either.
All advice I see online is about the SD card but is there anything I can do to test the board, or to drain static or suchlike?