DIAGNOSIS This is typically one of two things: a thermal overload shutdown or a failed trigger switch. The motor has a thermal protector that cuts power if it gets too hot. Let it cool for 15-20 minutes first.
FIX Step 1: Remove the battery and make sure it's charged (try it in another tool if you can). Step 2: Let the saw sit in a cool place for at least 20 minutes. Step 3: Reinstall the battery and try it again. If it works, you were just overheating it—ease up on the pressure and let the tool do the work. If it's still dead, the issue is likely the trigger. The trigger assembly is a common failure point and is a sealed unit.
IF THAT DIDN'T WORK If it's not the trigger, the next suspect is the brush holders or motor brushes. Open the saw body (remove the four long screws). Near the motor, you'll see two plastic caps. Remove them to access the carbon brushes. If they're worn down to nubs or the springs are broken, they won't make contact. Replace both brushes as a set.
HEADS UP If the motor overheated, feel the battery too. A faulty battery can also cause intermittent power loss. If the battery is very hot, let it cool down separately before trying to charge it.