We all know that the first step to troubleshooting Final Cut Pro is to trash the preferences file. You could go to Home User/Library/Preferences and delete the files manually, but why not use a really cool, and FREE, program to do it for you? Try FCP Rescue instead.
After you download and install, simply click the "Trash" button to throw away all of your preferences. Now remember: whenever you trash your FCP preferences you will need to double-check with our FCP 6 Quick Reference Guide to reset our recommended settings.