Someone stole a check from my granny and now they are making carpayments out of the account. What do I do?

Someone stole a check from my grannys checkbook and now they are making carpayments out of the account. Im not sure how they are doing it since they arnt actally useing the check they stole. How are they able to do this? It just comes out of the account. It has been happening for almost 3 months now and I just found out today. My granny is 90 years old and dont really understand whats going on. What legal action should I take?? They have taken over 3000$ so far. What charges will they face? Can we get the money back?