Just in case others ever have this happen, I have an iMac that I calibrated with a wide gamut for print repro a few days back and discovered (after a couple of hours of worried troubleshooting today) that shifting away from the default iMac profile will cause the built in monitor not to be found by fcpx. Everything will work but nothing will appear in the viewer. Hope this saves folks some worry/time

Weird... I'd shoot Apple some feedback on that one...

Thank you for posting this! After spending around 6 hours attempting to solve my problem, doing everything suggested in various threads (reinstalling fcpx, creating a new user, moving all of my media etc.) I finally stumbled upon this post, and my problem was immediately solved.

For anyone else searching the web for a solution, my issue was:

Using Final Cut Pro X on my MacBook Pro, all of a sudden the program would freeze anytime I tried to playback video. In addition, nothing would appear in the viewer window (including stills from the selected frame).

Changing my secondary monitor back to the default calibration immediately resolved the issue.

hey! you know what, I did the same thing again earlier this week and forgot my own gotcha post!
I did the whole nine yards until this niggle in the back of my mind finally dredged up this so simple solution:) Calibrated for extended Photoshop sessions then forgat to revert to the default calibration, when will I stop being an idioT?

That definitely needs to be reported to Apple.

I will, from within the app. Ive also posted this fix on the apple community discussion.

Hello there! I have also experienced this bug several times, and also forgot about it last week, took me a long time to get back to it! Anyway, it seemed it had disappeared for me after 10.0.5 but then upgrading to mountain lion seems to have put it back. It also has behaved differently on different occasions: sometimes the video monitor would just go black but everything else would seem normal, last week instead the application just froze and quit whenever i tried to playback any clip...It drove me nuts!!! I sent bug reports to apple and also wrote them in the feedback page. This bug is just unacceptable when you claim to have perfect color control with colorsync; if you can't calibrate your monitor, then what you're seeing is no good.
I really hope they fix this one soon and once and for all!!!