Happened to my merc, it was water that got in the (trim switch on the motor) prob when you washed it. change it, and you will notice the new one will come better sealed so water does not get in.

This has happened to quite a few Mercs. Not sure about other motors. Same as what Gaspard said.

the one on bow the one on gear shift or both???

on the engine. unless it doesnt have one then i would suspect the one on the bow.

Disconnect and bypass all the trim switches. Hook up one at a time and test it. If you have one on the bow I would start there. If the bow switch works properly disconnect it and move to the motor if it has one. Continue to the one on the console controls. I would have to say a switch has gotten wet or went bad. I had one on the bow of a bayboat do it and it sure was a surprise to see the engine trimmed all the way up in the boat shed when I know full well I had it down. I found it as the battery was dead as well. I was very glad the trim pump did not burn up. I am sure it ran until the batter died. Just another reason to install battery cut-off switches and use them. Likely one is the bow or engine. You will find it easily. Good Luck!

Captain Wilk