On this website: the primary navigation has all of a sudden stopped altering, i am certain i did not alter the core files. Exactly the same code works fine around the local machine, but please let me know whether it's just my computer and also you can't see what my issue is.

That sounds suspiciously such as the cache under Global Configurations...try that...I've experienced this problem before, underneath the Admin back-finish, the menus wouldn't work..on the other hand, I acquired the best results under Opera, in Chrome it does not work correctly...strange...

Hope this can help, Sincerely, Tom.

I believe the cache is easily the most likely reason as tommieb states.

Alternatively, maybe it's a CSS problem, since you are using sprites. You do not say what you are altering within the menu, but for instance should you change a food selection title from "Sports" to another thing, it'll still say "Sports" since the text is area of the image.