Fraud/cancelling unauthorised transaction?

I checked my bank account today and found it was $500 less than the previous day, I suspect one of my housemates may have used my Visa Debit card for such a transaction. I am going to call my financial institution in the morning to ask where the transaction took place and when, and if it was an internet transaction, or a signed or PIN transaction. I have never had this happen before (with my card anyway, previously she has hacked my Paypal account to purchase things over the internet...) and am unsure of the process for cancelling such a transaction. I will be cancelling my card and changing all my pwords and PINs. If it is an unauthorised transaction, is it likely that the money will be returned, or is it going to be something I have to absorb? Is there anything I can do about catching her, considering I may not be able to prove she processed the transaction?