Try restoring an older version by logging into your online dropbox account and selecting the ... next to the file in questions, click version history, then restore an older version. If that works, then yes, your file somehow got corrupted but probbaly not from somoene else being in it while you were.
Thanks for the advice. I'll have to put that in my back pocket for the next time it happens. As for this particular file, I had to close out with no chance of a save and then rebuild. No fun as it was a Chinese translation job, but thankfully only a couple of hours work.
Do you work or study in the creative industry? If so, which area?