Entire Numbers spreadsheet filled with question marks

I can see the words or numbers in each cell at bottom left of screen & I managed to change a few cells using Format Text. This was a document that I created last year & I wanted to just alter dates, etc. for reuse this year (after duplicating obviously). Unfortunately I can't seem to retrieve it properly. I had multiple versions & looked at some others - same problem.

First time this happened was just recently when I was sent a roster from someone & again everything came out in ??

I have Version 14.1 Numbers.