I can add something to Karen's problem. I too had this issue - unable to edit a document I had created, except for the headers, and having a blue background around my tables in the document.
I used Karen's solution - go to full screen (no change) then hit the 'Esc' key. Presto, suddenly editable again.
It was only after this I realised I had saved the document in Print view mode. It opened in the same mode - hitting Full Screen didn't change anything, but 'Esc' cancelled the print view mode, and allowed the document to be edited!
Hope this helps someone else out there!
Stephen
Numbers version: 3.6.2 on El Capitan