Can you not log in to Tiki at all? i.e. any page you try to access it blank? If that's the case you'll probably have to change things through the database directly and I defer to someone with greater expertise on that.
if you can log in and see pages but other users can't then it's most likely a permissions problem for certain user groups, specifically with the categories, since you said that is what you last change. Categories in version 2.0 now have 4 permissions associated with them:
You first have to make sure these permissions are assigned to specific user groups e.g. registered user group users should probably be able to view categories (tiki_p_view_categories) and view items within those categories (tiki_p_view_categorized), but you might not want them to be able to edit items within those categories (tiki_p_edit_categorized). The choice is yours.
Any new categories you create will then have the appropriate user group access assigned automatically (provided you leave a check mark in the box labeled "Assign permissions automatically:") when you create new categories. If you want to override the global settings then you can assign those four permissions above to specific user groups for each individual category. I think that for existing categories you may have to go back and edit them manually.