Q: FC no longer works with new OS?

Probably you just need to upgrade FC, don't you think?

I would believe Final Cut would require a update. Have you checked for updates?

Did you originally acquire the app through the App Store…or by some other means?

Russ

if you’re still on v10.0.9 or older, updates might not appear and you may have to do this:

If Final Cut Pro X, Motion, or Compressor updates don’t appear on your Mac

http://support.apple.com/HT204235

since media management changed a lot since then, be sure to review the links below, especially the youtube videos:

Final Cut Pro X 10.1.x: Updating and working with libraries FAQ

http://support.apple.com/kb/HT5958

MacBreak Studio: Episode 246 - Understanding Libraries in Final Cut Pro 10.1

http://www.youtube.com/watch?v=__wq1D9S-p0

Upgrading to Final Cut Pro 10.1 - PART 1 of 3

https://www.youtube.com/watch?v=GB93L8PEgxA

Upgrading to Final Cut Pro 10.1 - PART 2 of 3

https://www.youtube.com/watch?v=Yezi9DyaiuA

Upgrading to Final Cut Pro 10.1 - PART 3 of 3

https://www.youtube.com/watch?v=MybPaff8TZ8

What version is it?

Russ

When you lunch App Store icon doesn't it offer you to upgrade the app?

MMove the application to the trash and redownload it from the Purchases page.

I'm not either...

SElect it in the Applications folder and press Cmd-delete. What happens? Make sure there is no other mounted drive or partition with a copy of the application.