categories not working

Hi all,

I have a blog where the template displays the list of categories on the upper right hand side of the screen. However, when you click on one of the categories you get a 404 error, and the catergory does not display. I have ensured that there are posts made in each category, eliminating the ‘no posts to display’ dilema.
I am aware that there could be a few reasons that my blog is doing this, but I have no idea what they are or how to fix them! If anyone has any ideas what it could be, and how to fix it, it would be very much appreciated!