Unresponsive theme - nav bar breaks in mobile/tablet mode

Guys, please help.

You're breaking my heart here.

I'm now testing my site in all modes and I can see that it FALLS APART when I go to responsive mode. There's a bunch of white space at the top and bottom of pages and the nav bar is all over the place. I'm so gutted. Please help.

I'm hoping to be testing the site with beta users next week and at this rate I'm just not gonna be ready. I'm begging you here...can you help? Don't make a grown woman cry. And for god's sake think of the children!

Have a look at my screen shots and let's have a good old weep together. My site is http://www.hushhushvoiceovers.com

Hold me, WPMUDEV.

Emma