Starting over

I have a Near Coastal Masters which I have kept. Two renewals ago I also had a Towing endorsement with it. Some how the time has slipped away and I still have not Renewed the Radar Observer, and because of that the Towing endorsement has not been added to my license. My question is. Is the endorsement lost? What can I do to get it back. Do I have to start the Towing over from scratch. I would appreciate any help on this Thanks.