SupuS - check your HTACCESS file setup, as ALL pages currenctly lead to the same 404 page by the look of it. This could just be that the HTACCESS needs to be updated.

Stargre - the menu in the admin is stored in the database. Firstly, check that the database contains the menu if you can. This is can be found in the configuration_groups table. You then uses this ID to find all the configuration values within the configuration table, with this group ID. If there are no records here at all, it suggests that the SEO classes are not being called correctly. Which version of the SEO are you using, and can you post a link to your store?

Absolute