Which brand and type of battery are you using? If easier, you can send us a picture.
1. Remove the battery from the light and hold the light upside down. The factory usually puts a white insulating disc between the battery and the light to block the contact. It has to be removed before using the light. Please check the inside of the light tube and the back of the tail cap to ensure this disc is not trapped inside.
2. Install the battery and fully tighten the tail cap. Press the front side button. The light may have an ultra dim mode which is hard to detect in a bright room. When you tap the button, can you point the light close to your other palm to see if there is any light output? If it does, you can press and hold the button to adjust the brightness.