Ross, I'll bet you have two Numbers.App files on your hard Drive. I had the same problem, and here's how I fixed it.
1. Go to the FINDER
2. In the search box on top of finder, type in Numbers.app
3. At least two choices should pop up
4.Doduble click on one of them
5. Then try to open your file with that version of Numbers.app
6. If it works, then that's the version you want.
7. If not, then close out that version of Numbers, go back to the finder list and select another version. Then try to open your file again
8. If that version works then that's the one you want.
7. Trash all the versions that don't work. Then Quit Numbers.
9. Open Numbers from the Dock, then try to open the file in question. It should open with no problem.
In my case, there were two versions of numbers. One in Applications, and one in the Iworks folder, also in Applications. I hsve no idea how I ended up with the two versions. The Numbers Application standing alone and not in the Iworks folder was the latest and that works just finenow.
Good luck. BTW, this is the first time I'v tried to help someone in this Forum. It was fun...