Remove the part about AA batteries if the light is an MT1C/2C Pro.
For testing purposes, you can use regular Alkaline AA batteries so we can rule out a battery issue.
1. Unscrew the tail cap and remove the battery.
2. Check the back of the tail cap and inside the light tube. The factory typically packs a white insulating disc between the battery and the tail cap. Occasionally, this disc can fall into the tube and block the contacts. Make sure to remove this disc before using the light.
3. Inspect the threads near the end of the light tube. There should be only one O-ring at the bottom end of the threads. If an O-ring is dislodged or if an additional O-ring is installed on the threads, it can prevent the tail cap from tightening properly.
4. Install the battery with the button end facing the head of the light and fully tighten the tail cap.
5. Click the power switch. Does the light turn on now?
If the light works with the regular AA batteries, switch to the rechargeable battery and test it again.