PDF text disappeared

Hello! I was working in a PDF file and addind coments and highlighting some of the content. Suddenly, the initial text disappeared but my comments andhighlightings were still there. I also am not able to save that document anymore: Impossible to save the document. The file might be on the read-only parameter or opened by another user. Save the document under a different name or in a separate folder.

Then I click "ok"and try what was suggested but then it still doesn't work : Impossible de save the file. An error has occurred within the file. 

(I roughly translated from French)

Thank you ...