Admin crash after enabling Captcha

I had Drupal up and running and was galloping ahead with it until I installed Captcha. When I enabled it in administer>site configuration>modules I got a message saying that my space had been exceeded (too many bytes or something like that). Unfortunately I didn't save the message. I hit "back" on the browser and then most of the items on the admin menu disappeared. Now I can't even get access to editing my own account; nor to administer users, nor to create or manage pages, stories forum themes or images, nor to access the modules, blocks, themes, interface and almost everything else.

In short, I can't administer the site any more at all. The site itself is displayed just as it was before I enabled Captcha. I am logged in as admin, not as any other user.

To my inexperienced eye this doesn't look good. Any ideas what might have happened and how to solve it?