Check your cookies
The duck saves a quack in your cookies. Open your browser's cookie collection and remove the quack cookie for the site where you want to re-enable Quack Overflow.
For example in Firefox, right click on an empty part of the page, choose "information", then "security" and "show cookies". There should be a quack cookie with its value set to 1 for the sites where you disabled Quack Overflow. If you remove quack, Quack Overflow gets re-enabled.
Bookmarklet / code
If you don't want to check your cookies manually, you can also run the following code in your browser's console:
$.cookie("quack",null,{"path":"/","expires":3})
This will unset the quack cookie. Here's a bookmarklet:
javascript:$.cookie("quack",null,{"path":"/","expires":3})
Note that you will need to refresh the page afterwards.
References
Please take care when you follow the following guides, as wrong usage may make the Cookie Monster cry.