Hey Cecilia153,
Thanks for the question. In troubleshooting this issue, a great place to start would be to delete the user preferences:
Final Cut Pro 7: Troubleshooting Basics
http://support.apple.com/kb/TS1846
Remove receipts and reinstall Final Cut Pro
Another approach you might consider is reinstalling Final Cut Pro 7. To do this effectively, you need to remove the application and its receipts, then install Final Cut Pro 7 and use Software Update to install additional updates. You don't have to remove everything that was installed with Final Cut Pro 7. Follow the steps below to completely reinstall a fresh copy of Final Cut Pro 7.
Next, see if you can locate the media in Finder and attempt to open it with Quicktime. If the media is not damaged, and plays fine, you might need to reconnect it to Final Cut Pro:
Final Cut Pro 7 User Manual: Reconnecting Clips to Media Files
http://documentation.apple.com/en/finalcutpro/usermanual/index.html#chapter=92%2 6section=6%26tasks=true
Reconnecting Clips to Media Files
Clips in your project don’t have to be offline for you to use the Reconnect Media command. You can reconnect media files to clips in your project at any time, since all you are doing is changing the file path stored in the clip’s Source property.
Thanks,
Matt M.