F11 also works in IE to go to Full Screen or to toggle it off.

If that doesn't work, you can often get your Start bar to move to the front and become accessible by hitting the special Windows key on most PC keyboards made post-Windows 95. You should then be able to right-click on the tab that pertains to the window (or anything else that's open) and use the right-click context menu to "Close" the offending window.