You blog has been suspended. Check to see if you are in violation of [Tumblr's Content Policy][1].

> If you are found to be in violation of any of the below policies, you
> will receive a notice via email. Unless you explain or correct your
> behavior within 72 hours, your account will be suspended. We will do
> our best to work with you and ensure a fair outcome in all cases. We
> reserve the right to immediately suspend, without notice, any content,
> account, or IP address which we determine to be systematically
> generated, spam, or potentially damaging to our service or
> infrastructure.
> 
> When your account is suspended, public access to your content is
> blocked, and your Tumblr URL (and custom domain if applicable) is
> redirected to the Tumblr home page. Access to your Tumblr Dashboard
> and the ability to add/edit posts is also disabled.

Additionally, check to see if you received an email from Tumblr to the address you signed up with.

  [1]: http://www.tumblr.com/policy/en/content