Late last year, D&D Nexus reported that The Piazza moved from its normal URL to a temporary one. Now it is back on its main URL:
http://www.thepiazza.org.uk/bb/index.php

The Piazza, in case you don’t know, hosts independent forums for campaign settings for Dungeons & Dragons and other roleplaying games. It also has a section where D&D rules from the very earliest to the latest are discussed.

If you have any problems logging in, please post a message in the Facebook group for The Piazza and one of the moderators will give you advice:

https://www.facebook.com/groups/thepiazza/