Sounds like your project has one or more corrupt sequences in it, or other information in it is corrupt. Sometimes however, it can be due to other voodoo.
1. back up the project file, and the most recent version of it that you had in your autosave vault. Work only on copies that you make in the finder (Command-D to duplicate)
2. try copying it to another drive and opening from there.
3. try also deleting (backup first) fcp’s preferences and then opening FCP and then opening your project from another drive. look for “FCP Rescue” on Macupdate.com it handles this well.
4. try changing the permissions on the project file to 777 (all users get read-write-execute). Look for “Batchmod” on Macupdate.com to do this.
Further, there is or there used to be a key combination you could use when opening a project, so that it didn’t re-open any sequences you had, you might try searching for it, I don’t remember it. That way you could at least get the thing open, and sometimes it was a matter of trial and error to determine just which sequence was corrupt — as a last option scenario, you could delete that one sequence and keep going