If certain items vanish from your admin panel menu, this probably means you or another of your admins has changed your administrator permissions in a way that denied yourself access to various portions of your admin panel. If you still have permission to alter administrator permissions at Admin Panel -> Members -> Administrator Permissions, you can fix it there. If you can't get there, you will need to go into phpmyadmin. Go to the yourprefix_members table, browse the table and find your own member account. Edit your account's record and make the 'adminpermissions' field be blank.

Items will also vanish from the admin panel menu when you switch off features. Make sure you have everything you want activated at Admin Panel -> Settings -> Switches.