well go look for her, call the police if you can't find her. tell your parents (i'm sure you have already but yeah), ask your friends to help. and if your able to call her [like if she has a cell].
My sister freaked out completly because my mom figured out her fanfiction acount and she thought it was over. She ran out the door and that totally killed me because mom say it on MY computer. Well, they where driving out to find her about 2 minutes after her 'great escape'. Well, she actually hid in the house and just when my dad left to find her she ran upstairs and said that she hated me. I told my parents and they had an Hour talk with her. I know it's retarded to think she ran off, but she was screaming and hyperventaliating so..yeah. Well, there's my explanation. Just grand, isn't it?