Force Quit preview, and now it won't open the same document I was viewing all day

I was editing this file, and marking it up, highlighting etc, adding picture to it (as described in https://apple.stackexchange.com/questions/378888/add-images-to-existing-pdf-with-preview) , when suddenly Preview froze up and would not budge for a min or so. So I forced quit it, and then when I tried to open it again this error message shows up: > The file “...pdf” could not be opened. It may be damaged or use a file > format that Preview doesn’t recognise. Same message when opening in Adobe Reader app (which I downloaded to take a look to see if it's just a Preview issue). Did Preview corrupt my file? How can I get it back? It's pretty important for an exam and I don't have another annotated copy!!! Please help :(