ViewController controls not visible, but present in document guideline

I am newbie in Xamarin iOS development.

I created a storyboard, added few view controllers in it.
I added some controls in one view controller, ran the code and checked the output on simulator. All worked great.
Now after some time I checked my storyboard design. Now the controls are not visible in view controller(in story board), but they are visible in document guideline(view hierarchy).

please let me know what should I do to solve this issue.

Thanks in advance.