We've taught a LOT of people to use Drupal 7 over the last 4 years and one bug comes up more than any other.
When enabling large new modules such as CTools via the admin interface, it's not uncommon for other modules to become unpublished.
When those modules are unpublished, people often lose access for to the administration menu. Needless to say, that's not a helpful situation for beginners.
So, here's a quick guide to recovering from a lost Drupal administration menu.