Navigation Bar splitting over two lines

Hi folks!
I am an absolute beginner with Zen Cart but have successfully put together (after much Googling and following tutorials!) a site to enable my boss to offer light graffiti paintings for sale.
However, something has gone wrong in the last couple of days and I have been unable to find a solution to it, in spite of much searching. The top navigation bar, which was showing up fine, has suddenly started to split over two lines (please see attached screenshot). I made a small alteration to one of the items in the bar over a week ago and everything was OK after that. Since then, I have made no deliberate changes to it and cannot understand what has gone wrong.
The problem exists in Firefox and Google Chrome, though strangely it is displaying correctly in IE!
I am using V1.5.0. and the Bask Black Template.
The site URL is http://www.mouseonthemove.com
Any help would be much appreciated.
Thank you in advance.

Hello again folks.
I seem to have resolved this issue myself. It appears that, when I changed the title of one of the links, there was too much text in the new title and the nav bar was overflowing. I went into the CSS stylesheet and reduced the padding for navMainWrapper and navMain ul and everything seems to be displaying fine again.
Hope this may help someone else!

Thanks for posting the root cause and solution you found:)