I had a problem with a Keynote file: while trying to reopen that file, after normally saving and closing it, Keynote told me it was not recognized as Keynote file.Not recognized!??!
It was a 40 page document on which I spent something like 7 days… I was desperate!
Then I found this topic…
Here's the salvation!
1. Don't mess with original files - make a copy. Just in case.
2. You rename your broken FILENAME.key to FILENAME.zip. Finder will ask you whether you're sure enough on renaming the file.
3. Unzip the FILENAME.zip into separate folder to get folder named FILENAME.
4. You may spend some time exploring it - you will find all embedded pictures, videos and a PDF of your presentation inside, but all that is done just to clarify your curiosity without any practical reason.
5. Now you have to rename your folder FILENAME to FILENAME.key and here comes Finder again asking you. Do it. Your folder turns into the file FILENAME.key.
6. Now you can open it in Keynote, as if nothing happened.