I say lost, but more than likely stolen.

You need a scan tool to reprogram this type of vehicle. I was able to find what the sequence is to do it via GM.

Install a scan tool.
Turn ON the ignition (engine OFF.)
Select Body Control Module.
Select Special Functions.
Select Program Key Fobs.
If you are adding or are replacing a transmitter, choose the Add/Replace Key Fobs option. This allows new transmitters to be added without deleting those currently programmed to the module.
If you are replacing all transmitters or none of the transmitters are working, choose the Clear memory and Program All Fobs option. This prevents lost or stolen transmitters from being used to access the vehicle.