Were they located in your system drive? Or an external?
Does FCP X start, but you can't see your projects there? (This is how I understood your problem; if not, please clarify).
Projects reside in a folder name "Final Cut Projects".
In the internal drive, this folder is inside your Movies folder.
In an external, this folder is at root of the drive.
Don't move anything just yet. Locate the folder in the Finder and see if contains folders name after each one of your projects inside.
If they are, then FCP X should see them. Try clearing FCPX preferences - use Preference Manager, from Digital Rebellion (a free download). Preference corruption is the cause of many problems.
If the project folders seem to be missing in the Finder, then you may need to repair your hard drive.
If it is the internal drive, you'll need to boot from a different drive (you can use the recovery partition for that).
Use Disk Utility to Repair Disk.