- Open "Fonts" app check to make sure all the fonts are activated (This is usually it)
- Create a new user. Log in in to that new account restart iMovie. If this works you have a font issue as it relates to your user.
- Use the to Disk Utility -> Repair permissions -> Restart OSX
(or if you use OnyX, clean the system I usually use these settings)
If all else fails: Reinstall Mac OS. Yeah, I know...