Trashing iMovie's preference file may help. It can become corrupted in some circumstances.
Follow these steps:
- Ensure that iMovie is closed.
- In Finder, go to the folder <your user name>/Library/Preferences (where <your user name> is your Home folder). For users on Lion (10.7) or Mountain Lion (10.8), the Home Library folder is hidden. To access it, in the Finder menu bar click on Go while holding down the Option key.
- For iMovie '11, in the Preferences folder locate the file named "com.apple.iMovieApp.plist" and drag this file to the Trash. For versions prior to iMovie '11 (currently version 9.0.8) the preference file has a slightly different name. For example, for iMovie '09 the file is named "com.apple.iMovie8.plist".
- Open iMovie - a new preference file will be created with the default settings (this happens in the background while iMovie is loading).
- If iMovie now seems to be working OK, you can change the default preference settings as desired - click on iMovie's menu item "iMovie > Preferences" and make the desired changes for the relevant items in each of the tabs
If iMovie is now working correctly, it's OK to empty the Trash. Otherwise, return the .plist file to it's original location in the Preferences folder. Post back if you need further help at that point.