I have over 120 lift masters I maintain. You need to replace the remote. There should be a code on the back like 371LM. Get on amazon and order a new one. Reprogram the opener.
Thanks, Wulf. My wife and daughter both have a remote, we have the remote outside the garage door and my and my other daughter's vehicles have it programmed in. I would think that would rule out the remotes. I'll have to look into the reprogramming.