My friend stole forty dollars off my counter! She must have and i don't know what to do?

I am a very forward person so I won't be just sitting back or ignoring her or anything like that. But i had a friend over for lunch and i ran upstairs to get a project. When we get to school she says she has forty bucks and buys skittles and candy and stuff. When I get home my mom is going around crazy trying to find her forty dollars she left on the counter! What do I do? She has a history of stealing money and everything??!!