Numbers - “invalid file” error message for file created in Numbers...

I was working locally on a Numbers file on my iPad Pro over the past few days without a hitch and then, yesterday, noticed a message at the top of the screen stating that the file could not be saved on iCloud. That message disappeared and then, today, when attempting to reopen the file, I received the following message: “Couldn’t import spreadsheet. The file format is invalid.” I’ve turned off/on the iPad, checked for Numbers updates in the App Store...but still receive the same message. Any ideas on how to resolve this?

Many thanks!