The menu entry is probably still there, it just needs to be properly configured.

  1. Go to phpmyadmin.
  2. Go to jos_extensions and locate the Mobile Joomla! Entry. Copy it's ID number.
  3. Go to jos_menu and locate Mobile Joomla in that list.
  4. Edit the entry and change it's published status to 1 and change its component id number to the number you copied from jos_extensions.