@Bluegill1, Jim R. has it right.
One more thought: You were told when the loco is one the track powered, the wheels don't turn. Try turning it upside down and manually seeing if you can turn the wheels. There is the possibility that a piece or grit or something has gotten into the gears. Take an old toothbrush or small still bristle paint brush and try to wipe out the gear teeth. I've done this one in the past and it has worked for me.
So often I see videos and photos of folks trains, where they have extra engines not on the track, but sitting directly on the floor. Or worse yet, carpeting. This is a great way to get foreign substances, like dirt or grit, into the gear teeth.
If Jim. R.'s advice doesn't work, or if it isn't clogged gears, then mostly likely you'll need to have the board or remote replaced. Unless you swap out the LC board with a older convention type F-N-R board, but then they'll need a transformer in place of the wall wort. And of course, you will lose the sounds. Or you could strip the boards entirely, and run the loco on DC current with a power pack. Again, you lose the sounds and will only have F-R operation.
Personally, this is the one thing I don't like about LionChief, is that it's harder to diagnose and fix a problem. You have a remote, which if it's bad, means you'll need to replace it most likely with the Lionel Universal Remote.
And in the loco, you have only one circuit board that does everything. If something goes wrong, you most likely have to replace the board, which means going to Lionel direct. And that board may or may not be available.