Let Someone Use My Car But Now They Don't Wanna Give It Back

a while back I took out a car loan (that im still paying for) and let a family member use it until I moved out, I lived there in exchange of them using the car but now that I have moved out they don't wanna give it back and I'm stuck with the payments. they keep renewing the registration in my name and they insured it themselves, how do I get this car back? can I report it stolen? I'm also nervous I'm gonna get in trouble with the law because the car hasn't been registered in my state of residence for a couple years now. I've asked nicely multiple times, I've tried negotiating and they keep telling me "it's their car now" yet it's still on my name. do I have to do this thru the court?:/