Give the troubleshooting section a try and let us know what you find.
[[Milwaukee Hex Impact Driver 2656-20 Troubleshooting|Milwaukee Hex Impact Driver 2656-20 Troubleshooting - iFixit|new_window=true]]
You may need to open it up if you're handy with a volt/ohm meter so you can test the operation of the trigger. This guide will get you access to it for testing.
[guide|126797|Milwaukee Hex Impact Driver 2656-20 Trigger Assembly Replacement - iFixit Repair Guide|new_window=true]
Tell us the results of your troubleshooting and we'll see where to go from there.