404 Category Error on Joomla 2.5 website

I have a Joomla 2.5 website with 2000 pages. I use Search Engine Friendly URL's and want to keep them. Today, all of the sections under "Category" started to give me a 404 Category not found error message. However, all links under the menu options work, it is only those, which is the majority) that come from the category section that are giving errors.

I have not added any new programmes, the error only can after I added some sub-categories (as I have done many times before) and then selected "rebuild" for these new items to ensure they would show through on the corresponding page.

I am sure this is a relatively simple problem for someone who knows what they are doing. I have tried re-building the category tree, but this makes no difference.

This problem is very urgent.