Navigation block in header unusable

After having been fine before, the navigation block in the site header has suddenly stopped displaying correctly.

Here is the header in the Editor:

…and this is how it appears on the published site:

This frankly is making the site unusable. I’ve tried various changes to the header structure and changing the navigation block but nothing seems to make a difference. Help please!

The page I need help with: [log in to see the link]