Leaving a transmitter on until the battery pack is completely discharged will usually have the potential of only damaging the battery pack. However, if you're operating on a 27, 35, 40, 50, or 72 MHz channel with the transmitter antenna retracted, you could have damaged the output section of the transmitter. Your transmitter may need a trip to the service center.

If your transmitter is on the 2.4 GHz FASST system, no damage should have occurred, with the tranmitter quietly ceasing to operate once the battery pack became sufficiently-low. The battery may or may not become damaged.