Main Menu ok on mobile, not on desktop

Hello
Found no way to display the main menu on desktop screens
the code is :
<nav id="gkExtraMenu" class="gkMenu">

</nav>

nothing more, no <ul> <li> or <a> ...

no problems on mobiles , the menu is ok
what's going on ?

thanks a lot

Are you sure you have non empty menu selected as a menu in template settings?