Routine check: are you positive the empty file you opened was the same one you had saved, and not another location/version? What does the "last modified" field in the file's properties read? Does it match the time when you past saved?
That sucks. I haven't seen this problem before, and have no idea what went wrong. There should be an error message if the save process fails or if you try to load a broken file, I've never seen just an empty file before.
