Replying to Final Cut Express problem!

So i went to open final cut this afternoon and a little window popped up that says Licensing, and asks you to enter your first and last name, organization, and Serial Number. I downloaded my version of final cut 4-5 months ago, and I believe it's FCE4.0. I entered the serial number that came with it, and It says it's invalid. The program has been working perfectly up till today when this happened. Any advice would be great/ anyone else have this same problem?