Which brand and battery type are you using? If it’s easier, feel free to send us a picture. Can you please try the following troubleshooting steps:
1. Remove the pocket clip from the light if it's attached.
2. Unscrew the tail cap from the body tube.
3. Use a clean cloth to wipe the threads on both ends of the body tube.
4. Install the battery with the button top facing the front of the light.
5. Install the tail cap onto the body tube and tighten it completely. Most issues arise from a loose tail cap.
6. Press and hold the power button while pointing the light at your palm. The light has a dim mode that can be hard to notice in a bright room.
Does the light work properly now?