Diagnosis
This is almost always a failed trigger switch or a loose internal connection. The switch contacts wear out from dust and repeated use.
How to Fix It
You need to replace the switch assembly. Unplug the battery. Remove the four screws from the rear housing. Carefully separate the halves. Note the wire routing. Unplug the old switch from the motor terminals. Install the new switch (part 192059-5). Reassemble in reverse order.
If That Didn't Work
Check the carbon brushes. If they're worn down to the nub or stuck in their holders, the motor won't get power. Replace them as a pair.
Heads Up
When reassembling, make sure the wires are clear of the gear housing. A pinched wire will cause a short and can damage the electronics.