The 6-18240 is an "old school" Conrail Dash-8 with Magne-Traction and dual Pullmor motors, so it is certainly possible something was picked-up by one of the motors.
Since you've confirmed that it wants to move, flip over the unit and examine the gearing of each motor truck for any foreign objects.
If nothing is found, you'll need to apply power to the loco on its side or upside down. A pair of jumper wires with alligator clips on each end works well for this purpose. Connect one end of each to the center and outside rails of your track -- this way your command signal will still travel to your loco. One of the remaining wires should connect to one of the roller pick-ups, while the other can connect to the motor truck "structure" just about anywhere that's clear of the wheels for grounding. (The motor truck and locomotive frame are grounded on these units.) Apply power, and address the locomotive as you normally would with your handheld unit.
Both motors should start moving at close to the same time, with a one or two speed step difference not being a big deal. If one motor doesn't respond properly, you've found the issue.
I should also note that given the age and mechanical make-up of these units that the hardening of lubrication contained therein can sometimes be a problem. But given you mentioned that this problem started "all of a sudden," I'm thinking as MartyE did above with a problem in one of the individual motor trucks.
TRW
