Help! I'm not positive but i think my crayfish is dying!?

My little sister is in fifth grade, last year she brought a crayfish home from school, (for science) and she took care of it as a pet. She wasn't so good at cleaning the tank, but she did it once every two weeks. We have had him for about a year now, and last night i was doing homework, and i checked on him. I noticed that he had shed, and when i saw HIM he was laying on his side, and his side of his shell seemed week, and sort of caved in. My sister said that is because he just shed and his new shell is soft. But he has been on his side all day, and he still is. I don't think he has any strength to pull himself up, or move around. I'm not sure if he is dying or it's because of his shedding. Also I don't know if they can drown, or dry out, but i want to make sure he has JUST enough water, and not too much or too little! could you please direct me! I am afraid he might drown or dry out! I don't know what to do, and I can't just let the poor thing suffer!