try to think where she goes when she wants to be alone
Call the police, meanwhile gather some people to go find her -- first search some places she tends to go, then look deeper.
If that makes sense.....
I assume you guys called the police already? I would drive to spots she likes to go and look for her. I think they have that 24 hour missing thing. But, hell if it would stop me from looking before that.
Call her parents or the Police