Acrobat Reader DC: I can't re-open a saved file, error message

For the past 2 days, I've been working on a PDF and regularly saving and re-opening it. Last night I saved it, as usual, but this morning, when I tried to open it again, I got the following error message: "Acrobat Reader DC could not open [filename] because it is eithere not a supported file type of because the file has been damaged (e.g., it was sent as an attachment and wasn't properly decoded).

I tried opening a previous version but was told there are no previous versions available.

Thanks.