Mark is thinking the same thing I am: oxidation. Trains stored in a humid environment can suffer from poor electrical contact once you start them up after a very long break.
If every other locomotive you have is running fine and you have been running trains regularly in the past two years, the change would be in the locomotives themselves. Clean the pickups and the wheels and see if that helps.
If you haven’t run any trains for two years, clean the track.
It all depends on the details of your environment.
