You need to meet with a landlord/tenant lawyer in your area to discuss your options. You can sue the tenant (if you can find him) in small claims court. Or, you can sue with an attorney in county court. You also can check into whether you may be able to retain your security deposit.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
If you have a security deposit, you need to make sure that you properly send notice to the last known address explaining the losses and damages you are applying the deposit to. This is very important as messing this up may result in you owing money to the tenant even though he did not pay you as agreed.
After that, you need to do your best to mitigate your damages (find a new renter) and then you will have to pursue the tenant for the money owed to you. You can file a lawsuit and obtain a judgment or you can hire a collection agency to find him and push for payment. Be careful how much money and resources you put into pursuing what you are owed as people who do not pay their rent often have little ability to pay. You may just have to write off the missing money as a loss.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.