I had this happen once. Short answer: I discon/connected the FNR switch/harness and traced the harness to ensure no shorts or fubar'd connector pins. Plugged everything back together and it worked.
IIRC you can check the disconnected FNR switch with a simple continuity tester. If you have a service manual, you'll know which pins to probe right off the bat; otherwise, it's trial and error. (I have the SM but am presently at work.) What TC revision to you have ? (ie 'A'...'F') Also...I think my machine displayed a "code" when this happened.