No, not unless you've made some provision to back up the old version.
For example, if you had checked the option "Always create backup copy"
in Tools Options Save, there would be a copy of the previous
version in a file named "Backup of YourDoc.wbk" in the same folder.

If that option wasn't checked, you could still get back the previous
version *only if you haven't already closed the document* by clicking
Undo to remove your changes. Once you close the document, though, the
Undo memory is gone.

See http://www.gmayor.com/automatically_backup.htm and
http://www.word.mvps.org/FAQs/General/AutomaticSave.htm for
explanations of some other things you can do to prevent the same
problem from happening again.

--
Regards,
Jay Freedman
Microsoft Word MVP FAQ: http://word.mvps.org