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 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 :(

Yes, Preview may have corrupted your file. If you have been 'saving' it continously, there is one thing you can try -

1. Open the file in Preview.
2. When you get the error message, click OK to dismiss it.
3. Then cick on File > Revert To > Browse All Versions... on the Preview menu bar.

4. This may show you all the previous versions of your file. Use the slider on the right-hand side to browse the previous versions of your document.

5. Choose one of the previous versions of your document. Then click Restore (or hold the Option key, then click Restore a Copy to create a duplicate file).

Depending on how corrupt the document is, this may or not work. (More info - View and restore past versions of documents on Mac).