Authentication Required pop-up

In recent days an “Authentication Required” pop-up window has taken over my blog requiring all visitors to enter a username and password. I upgraded to WP 2.5.1 in early July but this problem didn’t appear until last week. Also, my RSS feed became invalid at the same time.

Does anyone know what could have caused all of this? Is it some kind of comment spam attack that infected my blog? Most importantly, how do I fix this? How do I get rid of this “Authentication Required” pop-up?

The URL of the blog is: https://www.codyscuentos.com

Thanks in advance.