My existing categories

When I try and add a new product, I have the choice of adding to existing category or add a new category. my existing category list is gone... How do I get it back?

Unless you have a devloper who can help you, I recommend you submit a support request so that EI can have a look at this.  There are questions I'd be asking such as "are the categories still in the store" and "have you altered anything" but someone who knows the application needs to get logged in to the store and look at stuff.