Your description is a bit hard to follow because what you describe typically
doesn't happen. If you've opened a file, made changes & haven't saved and
then - while the file is still open - you go to File> Open & attempt to
reopen the same file it *doesn't* revert to its original content... you just
get dumped right back into the open file as it currently stands.
I think it's more likely that after making the changes you *closed* the file
without saving, then reopened the file later. If this is the case there is
no "new version" to get back - any unsaved changes were vaporized when you
closed the file.
If I'm misinterpreting your circumstances please reply with a more accurate
description of exactly what happened.