The reason the forum was down last night is explained in the statement below copied from Forumotion's Facebook page today.
Other forums were affected too.
Cyber attacks like this are the reason we'd already decided to give anyone who hasn't logged in for a year warning of cancelling their membership today.
ie removing unused registrations and the email addresses that go with them is good cyber security housekeeping.
It is also why the apparent number of members looks a lot smaller today but in reality we'd a lot of silent registrations.
Forumotion statement:
Hello,
As you saw last night we had a temporary unavailability of some of our services caused by a major attack that had technical repercussions.
While our teams reacted quickly, we could not do otherwise than to go through it. Our infrastructure is designed to absorb such attacks but could not absorb all the excess traffic which explains the inaccessibility of your forums.
We had to perform maintenance on much of our servers and restart the entire infrastructure.
Our teams have worked much of last night and almost overnight to address this concern which we were undergoing as much as you.
Thanks to this work no data loss has been recorded.
Currently, everything is back to normal but we are still vigilant. 
We apologize for the inconvenience.
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