As the document has been closed you cannot return it to the way it was,
unless you have a backup copy.
This is probably a corrupting document. Several options to try:
1. Open and Repair, in the File > Open dialog, there is a downward-pointing
arrow on the right end of the Open button. After choosing the file, click
that arrow to get a small menu. Click "Open and Repair" in that menu.
2. Copying all but the last paragraph mark into a new document.
3. Open new blank file; Insert, File.
Work on a copy, just to be safe.
Are you saving this document to a media other than the hard-drive?
I have come across this message but only when the files has been opened and
directly saved onto a memory stick. Never work (i.e. open/edit/save/etc.)
with Word via a removable media, always save to a hard-drive, do the
necessary, save/close, copy to media.
Come back if you still have problems.
Hope this helps
DeanH