What can I do if a person is in possession of my car and won't give it back? It is registered and insured in my name. Am I responsible for any tickets or accidents that might occurred even though I am not driving it?
You could go to court for a court order compelling them to hand over the car. Or, if you can do so peacefully, you can go and get it. But first why don't you tell us how this happened.
File a police report for the 'theft' or whatever they agree to call it.