I'm also facing a similar problem after I dropped my Nexus One phone from 3 feet on concrete floor.
Initially nothing worked. All the 4 touch controls, Proximity Sensor, Camera and trackball weren't responding.
Then I took backup and reformatted the phone. After reformatting, 4 touch controls and trackball started working.
So issue is with Proximity Sensor and Camera.
I installed an app from Android Market to test the working of Sensor and Camera, Reading in the app showed they are dead.
@Keith Fawcett : Thanks for your answer. Do you have access to some tutorial or Video which you have come across, which will help me fix this issue myself. If you can provide me something where and how to fix Ribbon Cable it would be great.
Thanks.
P.S: I have checked lot of Video on YT but most of them are for broken screen or broken power button. Didn't find any on this Ribbon Cable though