I have a problem with my theme. When i am in IE8 the menu

HI
I have a problem with my theme.
When i am in IE8 the menu disappear, as in the mobile version and it becomes and small square in in the top left side of my screen. But the problem is that i am in the desktop version.
I have to click this button in order to my menu appear.
It just happen in IE8, i have tested inthe rest of browsers and IE9 and everything is ok.

I guess it will be a css issue but i cant handle it.

Could you help me please.
I'll send you a couple of screenshots.
Thanks