This generally happens when you change the project file's name with the file actually open. It can also be the result of a corruption. The trick is not to close that file until you've ascertained that you've rescued all its content.
First try a Save as to a different file name in a different location, preferably in your internal drive. Open that file and see if it's okay. If so, close down FCP, dump preferences, repair permissions, and restart the computer. If Save as is greyed out, copy all of your project file's contents into a brand-new project file, then save that -- again, to a new location under a new file name.
These file errors can mean nothing, or they can mean everything -- a corrupt project file, if it overwrites your good project file, can wipe out everything you've done. Have you done any manual backups up to now?
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