My friend is hiding something from me?

My friend is a cheerful bubbly friend who everyone likes to hang out with but i have notice that recently she has become sad and quite. She won't talk to anyone and she is drinking and since she went missing for 5 days she dosn't want to hang out anymore in or out of school. She says nothing is wrong and she was "fine" when she went missing. We have notice that she won't let anyone touch or go near her side (by her ribs/stomach) and i think she is hiding something from me because she won't talk much and very jumpy like a nervous reck. What should i do?