I am having this same issue. I have individually deactivated each of my plugins and that did not change anything. I also renamed my entire plugin folder as to not be seen, and switch to the twenty eleven theme, nothing has changed.
All of my categories are gone and I cannot create anything new.
To the left of the post entry's there should be a list of categories which are longer visible. When I attempt to create a new category to replace the missing, I am told it already exists. http://www.bitcentral.com/blog/
Any help is greatly appreciated.
Thank you.