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When enabling large new modules such as CTools via the admin interface, it's not uncommon for other modules to become unpublished.
When those modules are unpublished, people often lose access for to the administration menu.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.
This is a screenshot showing the administration menu:
If you enable some modules and then the administration menus disappear, the image below shows what you'll see.
Notice that there are still signs that you are logged in (I've marked those in red). This is important - if you are logged out, you won't see the administration menu either, but that's a different problem.
What you need to do is access the main "Modules" page. The best way to get there is by typing in the URL of the modules page.
Try adding one of these to your main domain name:
Now on the Modules page, you need to re-enable two modules:
Check the boxes next to those modules and then click "Save configuration".
Once those two modules are enabled, you'll have your administration menu back.