I just spent three hours solving this issue. More than likely your Navigation Controller in the xib is messed up and the bar is not "connecting" to the application.
So, in IB, delete the existing Navigation Controller, and add a fresh one from the Library. Ctrl-drag from the Delegate (NOT the File's Owner) and drop onto the new Navigation Conroller, and select the delegate's outlet (you usually call it navController or navigationController). Then apple-4 and change the Class of the underlying View Controller (Root View Controller) to the name of the main View Controller. If that view controller has a nib/xib, specify it by pressing apple-1 and choosing the NIB. Again, this is for the View Controller, not the (parent) Navigation Controller. Save and re-build.