Yup, this has happened to me in the past (in the Bahamas). Turns out ATT basically shuts off your service to save you from unwanted high charges (HA!).
To get started, I unlocked one of my old ATT phones (it was a blackberry I had lying around). If you call ATT customer service they will walk you through the process. I then brought it to the Digicel store by the Burger King, bought a SIM and some minutes, and I was good to go. The staff at the store even installed the SIM and made sure it worked before I left. Alternatively, you can purchase an unlocked GSM phone from ebay or even get a cheap phone right at the Digicel store in Negril. However, if you wait until you get to Negril, they may tell you the cheap ones are out of stock and you'll end up paying $50 anyway.