Many state have requirements that credit balances be turned over to the state, as unclaimed property after a certain period of time, if not claimed.
In fact, a while back, I received such a letter from my bank credit card dept., saying that if I don't request a refund, that is what will happen. You'll still get your money, but at that point, you'll have to file a claim with the state.
You might have just missed the notice.
In fact, the legal section of newpapers here are filled with notices of unclaimed property. One thing I don't understand is why they just didn't just send me a check.
I suspect the state just needed the money, and hope people like you just forgot about it. As was suggested, check and see with the bank first.

