I lost mine a while back. I just had to goto the dmv and get a new one for like 5 bucks. If you tell them you think it might have been stolen, then they will tell you what you need to do. If whoever stole it hasnt sold it to themselves, maybe you can get a new one issued with a different registration number or something.
All i had was my drivers license and they had my car on the computer. DIdnt need a blood sample or any other extensive proof that I was who I said I was.
EDIT: this probably doesn't need to be said but do it sooner rather then later ;P