DIAGNOSIS This is a circuit interruption. The most likely cause is a failed trigger switch assembly, part number 02-06-2821.
FIX First, rule out the battery and tool terminals. Test with a known-good M18 battery. Visually inspect the battery terminals on the tool for debris or damage. If the tool is clean and the battery is good, the trigger has failed. This requires disassembly and replacement of the trigger switch assembly.
IF THAT DIDN'T WORK The secondary cause is a broken wire or connection between the trigger and the motor controller board. This requires full disassembly to inspect all internal wiring harnesses.
HEADS UP The trigger assembly is not a user-serviceable part under warranty. Attempting to repair it yourself will void the warranty. Contact Milwaukee service.