Is your Camera set up and Installed? - Follow our setup guide
Camera Cable Connection - 90% of the time re-seating the cable fixes the problem (as the fiddly small cables only have to be slightly out to fail). Try removing and fitting the cable again.
Cable Direction - Another common issue where the metal part of the belt isn't round the right way which means there will be no connection at all. See our setup guide for pictures.
Is the Camera Enabled? - Have you checked that the camera is enabled in the raspi-config settings (sudo raspi-config)?
Is your Software up to date? - Run 'sudo apt-get update' then 'sudo apt full-upgrade' in a terminal window to ensure your camera has the software it needs.
Are you using the most recent version of Raspberry Pi OS - Which is 'Buster' at the time of writing. If you're using 'Jessie' or another previous version, you will need to reinstall the SD image, especially when using the HQ camera.
Reboot - Try rebooting your Pi if you've changed the camera whilst it's switched on (we'd recommend turning it off before switching the camera cable though).
Operating System - Are you using the Raspberry Pi OS? The camera module is only officially supported by this operating system.
If your camera module is still showing no signs of life, please contact support.