Ok I heard that if you COMPLETELY take the tamagotchi apart and let it sit for a couple of days to dry out and then after about 2-4 days have an adult help you put it back together. If it still doesn't work, sorry. Maybe you could contact Bandai and maybe you couldn't return it and switch to get another one.
REMINDER: Have an adult (if you're not an adult yourself) help you take it apart and put it back together. Remember, after you unscrew the back part that leads to the battery, there are, I think, 3 or 4 more screws and you unscrew them too and then set out all the pieces away from little children (maybe put them on a paper towel on a high shelf) and after 2-4 days have an adult help you put them back together (if you aren't an adult yourself).
Hope I helped!
*~Tamagotchi~*