Numbers file can't be opened

My MacBook Pro (early 2011) recently had a meltdown and the OS was restored at an Apple shop.

Thankfully, the technicians were able to retrieve my files (not all of which had been recently backed up), but some of my Numbers spreadsheets will not open and I see the error message '"filename.numbers” can’t be opened.'

The computer is currently running version 3.2.2 (1878) of Numbers.

This doesn't happen with all of my files, just some. Annoyingly, all the documents I need the most recent versions of are the ones that don't seem to open!

I'm currently running Mavericks on my computer. I've yet to update my computer to Yosemite and the newer version of Numbers, so don't know if that might fix the problem?!

Does anyone have any ideas how I open these files I so desperately need?

Many thanks!