Most of the time, you have just turned off the light accidentally. For Mac users, press the F5 to reduce or dim lighting or Fn+F5 for Windows users. On the contrary, press F6 or Fn+F6 to increase the lighting.
Another scenario is the keyboard is just on low battery and automatically turns off the backlight to save battery. In this case, charge it and then follow the above instructions.
If the keyboard continues to not light up, please contact us at support@keychron.com and our team will reply to you within 48 hours. In the email, please attach a video or pictures of the problem you're experiencing and your order info so our support team can help you as fast as they can.