Maybe...
If you haven't saved the workbook, you could save the file you're working on as
a new name.
Then open the original (and this renamed file) at the same time and copy/merge
the worksheets the way you like.
If you already saved the workbook and you don't have a backup version, then
maybe you can talk to your IT staff to see if they make backups that you could
get restored. Check with them to see if you have to rename the existing (bad)
one so that it doesn't get overwritten with the old (better) one.