I am assuming you are saying you were flying DL BOS-ATL-TPA and the ATL-TPA flight was cancelled . Instead of waiting on another flight you drove.
You definitely should not only still have a flight back, but are owed a refund for the missed segment (probably about 2 bits) and likely thrown some miles as a gesture.
I would call back at some time to discuss the refund and compensation. That will help insure they have the return flight confirmed.