You are welceom to take it our of the their deposit if you have anymoney left over from the rent they owe you. Or you can take them to small claims court. You can call them and demand payment for it base on the value that it was. or to return ti outright.
I have to agree with Arlan, hopefully the rest of the house is in decent condition. You can usually pick up a newer fridge from Craigslist for a couple hundred bucks. Look for the people who just did an upgrade and only got rid of the fridge cuz it didn't match.
A resident did that to my family once, took all the appliances when they left. They were such a pain that we just moved on.
Make a police report, suspect known. Hopefully, you have the serial number of the refrigerator on the move-in checklist that the tenant signed on move-in. I have also started photographing the serial numbers of my appliances in my pre-move in photos. Put a post on Craigslist stating your make, model and serial number of the refrigerator, and that it is stolen property, and should be reported to the police.