1) Make sure that the flat flex cable is laying completely flat under the leg of your iMac. ( it might have got twisted in the installation process .)
2) Plug the cable directly to the back of your iMac. ( avoid any USB Hubs )
3) Disconnect any external devices (hard drives) from your iMac/Thunderbolt Display.
4) Try different ports on the back of your iMac
5) Reboot your computer.
Disclosure: Can’t guarantee that it works with older Cinema Displays and iMacs before 2009