300ex wont start!!!

My brother was riding the 300ex and when he came home i took it out nad noticed it was bogging out or cutting out when you gave it full throtle....never did it befor now today he went to ride it and it just bogged out and stalled he jumped it but everytime he put the clutch in to shift it stalled. now it wont start and its stuck in reverse...you can shift and it acts like its in gear but when you put it in neutral the revers light goes on....it wont start......please i need help.....could it be due to water or mud in the carb. or something like that anything helps. thanks