[Mark Freed]

I had this problem too. In order to get back into FCP, you should dump your preferences by going into the folder Library/Preferences, and then deleting the file com.apple.FinalCutPro.plist and the folder Final Cut Pro User Data.

Then, once you’re in FCP, you’ll have to reset your preferences, including the scratch disks. What caused the problem in the first place, at least for me, was that my Thumbnail Cache scratch disk was set to a remote drive. Make sure this in particular is set to a folder on the MacHD, and FCP should open normally again.