Hello,
The issue is definitely that you are running out of horizontal space and reducing the font size would be the only solution to resolve this if the other ones aren't an option. The reason the menu shows up "blank" is because it is being pushed to the next row which is off the menu block. If you add this to the s5_responsive.css it should help get the menu to show up on smaller screens.
@media screen and (max-width: 1250px) {
#s5_nav li { font-size: 0.8em }
}
@media screen and (max-width: 1150px) {
#s5_nav li { font-size: 0.6em }
}
-Mike