Check the wires to the decoder. Is it possible something has come loose? Is the motor properly isolated from the frame? Also are the right and left pickups isolated from each other? Using a continuity tester on a multimeter will be able to help you allot.
It is possible that the DCC decoder went bad, but the most likely
cause of the loco short circuit is as suggested by tkruger. Take
the shell off and look around. The type of frame and it's wiring
will be different from model to model. Tell us the loco make
and model and the type of frame. Some are cast metal, some
are split frame.
If you don't find any wiring anomaly, you can determine whether
the decoder input is shorting by disconnected it's red and black wires,
then feed them direct from the DCC controller. If you then get a short,
the decoder has in fact gone bad. If you installed the decoder you
may get a free replacement by contacting the maker.