Backing out of registration produces blank site

After installing captcha on my test system, captcha failed to produce a viewable graphic. The response was then to back out (using the browser back button) of the process.
Doing so produced a blank site, in that Drupal failed to produce any content, other than the housing html and body tags.
If you have captcha working, you might want to try backing out of the registration process and see if you can reproduce the same error?
Thanks for any help/ideas.