Assuming you're asking about the address bar in Windows Explorer, if all of the bars on top are gone (full screen mode), hit the F11 key.
If you have any toolbars showing, you should be able to right-click on one of them and select address bar again. If View/Toolbars/Lock the Toolbars is selected, you may have to unlock it first to make any change then relock it.
If none of the above works, run a spy sweeper program like SpyBot S&D on the system.