DIAGNOSIS This is a motor or controller overload and failure. The motor controller board (part of the field assembly) has likely overheated and failed, or a motor winding has burnt out.
FIX Let the tool cool completely. Attempt to use it again. If it is dead, the controller or motor is fried. This requires electronic diagnosis. The most common fix is replacement of the motor and controller field assembly (02-06-2820).
IF THAT DIDN'T WORK Check for a seized bearing. A seized bearing on the crank or counterweight will lock the mechanism, causing the motor to stall and burn out. Manually try to turn the reciprocating shaft. If it does not move freely, a bearing has failed.
HEADS UP The burnt smell confirms electrical damage. Do not continue to install batteries or pull the trigger. This is a serious failure that requires professional Milwaukee service.