Keith Fawcett is right. The ribbon cable has most likely become detached causing the screen to go black during a call, your camera to not work, and/or the sensors to not work.Promity.
My nephew and I opened my Nexus One and we were able to fix this issue. Here are some videos we used to open the Nexus One. Willy Castillo and Steven Vilchez from Managua, Nicaragua.
http://youtu.be/dcEspM1Dkzo (1/3)
http://youtu.be/WrSzdZSgkQc (2/3)
http://youtu.be/eiIn1ta2lb8 (3/3)
[guide|1654] (photos)
Other link to open Nexus One: http://www.youtube.com/watch?v=Mx3oRsWAQjo