Check to see if self.navigationController is nil.
If it is nil that means that you are not running within the context of a UINavigationController (the system sets this property for you when the UIViewController is added to a nav stack).
If this is the case then you have not properly set up a UINavigationController.
Note that you can not set the navigationController property yourself. The systems sets it for you when the UIViewController is added to a UINavigationController's stack (and sets it to nil when it is removed from the stack).
To set this up you will usually create a UINavigationController instance right after you create your main view controller.
UIViewController *mainViewController = ...;
UINavigationController *mainNavController = [[UINavigationController alloc] initWithRootViewController:mainViewController];
// now present the mainNavController instead of the mainViewController
If you are using storyboards you would drag out a UINavigationController instance and replace the default root view controller with an instance of your mainViewController.