This can happen sporadically. To ease the following description, lets just talk about the rear shifter/derailleur.
You get water all over the place and it freezes. You are able to shift to bigger rings by pulling cable with the shifter. Then when you attempt to shift to smaller rings nothing happens.
The force of your hand shifting in one direction breaks the ice and it works. Shifting in the other direction is done by releasing cable and relying on the derailleur spring have to pull the slack and move the cage. The force of the spring may not be enough to break the ice and you just get slack in the cable but the derailleur doesn't move.
Possible solution: Get off the bike. Change gears at the shifter to go to the smallest ring. Now pull the derailleur cage with your hand to break the ice. Once things start working again, keep shifting...even if you don't have to. This will keep things from freezing up again.
After going through cold water, it's a good habit to shift the gears and pump the brakes. If everything just sits there motionless, it is more likely to freeze.