Undo "merge changes"?

[Lori]

When my boss opened a document, it asked her if she wanted
to merge changes. She hit yes by accident, and the new
document overwrote the one she needed which happened to
have the same name. I need to figure out how to restore
the document before the other one merged and deleted it. I
can't believe that this has never happened to anyone else,
and hope there is a fix.

Thanks,
Lori