It happened the last time I washed the bike. It's happened again this time. The rear derailleur won't shift the chain up even though I was careful about not pushing the cog wheel up or down. Last time the bike mechanic sorted out stuff but I can't do this every time I wash the bike! What has happened and how do I re-adjust the derailleur after every wash?
This is an interesting one - if we treat it as a derailleur that won't move right, perhaps work backward to the cause, instead of working forward from "where could water go to cause this?"
Start by cutting the problem in half. Suspend the bike from a work stand or hang it up with some rope. Pedal with one hand while hauling on the wire that runs down the downtube. You should see one derailleur move. If its the wrong one moving, pull the other cable. As you add tension, the rear derailleur moves closer to the wheel/an easier gear. The front derailleur goes the other way - gets to a harder gear as you add tension.
The point of that is to see if the issue is in your shifter lever up the top, or the derailleur.
Next is to gently push the derailleur with a finger while pedalling slowly. Do watch out for finger-eating gear bits - possible damage can range from "ouch" to "I can only count to nine and my bike is painted blood red" You are duplicating the action but taking the cables completely out of the picture.
Why? Cables can rust up and mess with shifting. Going around them can show that.
Finally, take some decent pictures of the rear cassette area and post them in your initial question. Then we can go a bit further.