categories disappeared

Hi,
Im currently building a new site on the back of a current domain whilst waiting to move the domain. anyway Im building it using wordpress 2.5.1
all was fine as normal when suddenly when clicking on each category brought up a 404 error message.

i went into the admin just to see if i had changed anything but everything was fine, went back to the front end and there was no categories there although the posts were showing on the homepage, the tags have also gone.

I did try adding the categories again but they kept deleting whenever i went back to the homepage.

very strange take a look
http://kreativdesigns.co.uk/ramology/