In NYA, there is a presumption that ownership is in the person in whose name the vehicle is registered although such presumption is rebuttable. You have the certificate of title, so you are in a pretty good position to insist that you have ownership of the car, especially if the auto is registered in your name. So, if you want the car back and she won't return it, you could report the car as taken without your authroization or even stolen by her. The police would most likely take the complaint.
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You can. You also need to make sure there is still insurance on the car. If not , and the plates have not been returned, your license will be suspended. You can report the plates stolen as well.