Hi there,
I'm a Mac user, but at one point had this problem as well. What I tried doing was quitting out of Opera as a whole (in your case, going to task manager and ending all the Opera processes) and then starting it up again. When I did that, the back button started working again. I'm not sure about Windows, but I would try the ol' reliable close-and-reopen process anyway, just in case.
Hope this helps.
~~TSwany