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My ex sold our car that still had money owed 
It is a legal issue for two reasons. In the United States if both names were on the title both people would have had to sign the paperwork in order to transfer the title. If the car was collateral for the loan, then the bank would have had to be involved in the transaction. The portion of the check need to repay the loan would have had to have been made out to the bank. If the car was sold to a dealership, then paperwork must have been forged. If the car was sold to a person then it is possible that they were too naive to know what paperwork was required, but it is likely still fraud. You need legal advice to protect your money, and your credit score. They should also be able to tell you who needs to be contacted: DMV, the police, the dealership, the bank.