As of about an hour ago, Ubuntu Forums is once again available to the general public. It has been down since the Saturday before last after a hacker managed to use a compromised forum administrator's account to deface the website.
One important thing to note: you can only log in via Ubuntu SSO (Single Sign-On) at the moment. If you had a forums account but don't currently have an Ubuntu SSO account, register for one here and then set your preferred email address to the one you used in your forums account.
And now that the forums are back up, Canonical has posted some more details about what exactly happened. In summary:
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