Hi Ishmikh_1
You need to set your mobile menu. For this,
Go to,
Wp-admin > Appearance > Menus > Select your main menu >
Tick all the Mobile menu option there.
For Reference: http://prntscr.com/low3ty
Please let us know if this helps you.
yes I do remember this issue .
I debugged this to the core files but I was not able to find the reason behind it what the wordpress core function " wp_nav_menu " is not working in your site.
We would need some time to check this issue as its a really really strange issue and never been reported and experienced before.