Magento

I was adding some new categories to my store when the back end of the site froze up. Now I can’t log in to the back end and the store itself does not display, either. All I get is a 404 screen with a Magento Demo Store header and a “Page not found” message. Is there another way back into the store? I am/was running 1.16. Since the Demo store header appears, what happened to all my data?