Navigation bar disappeared from my storyboard items
Try this:
Select the view controller with the missing navigation bar and view the attributes inspector (This is the fourth tab over in the right side window). Under the simulated metrics section, change the "Top Bar" from the default "inferred" to "Navigation Bar".
Hopefully that solves your problem.
Specifically for xcode 6, under the simulated metrics section (fourth tab on the right window), under the "Top View" dropdown, pick the "Translucent Navigation Bar" to get the default nav bar.
For me I just command-A and copy, delete all view controllers, then paste back in, and it worked.