The control switch is locked. It's in the center position.
Check the switch on top of the tool, above the trigger. Push it to the left for forward or right for reverse. The trigger won't work if it's centered. If the switch is already in forward or reverse, the battery isn't seated. Remove it and slide it back in until it latches. Make sure the release buttons click.
If that didn't work, the battery is dead or faulty. Try a different, fully charged M18 battery pack. Don't use batteries from other brands.
Heads up: Always lock the trigger or remove the battery before changing bits or storing the tool.