The site went down for several months due to technical problems.
Several months? This is the root cause of your problems. You cannot allow this to happen and not expect a kickback. What technical reasons did you have for letting the site go down for all this time?
This will not help and it will sound unsympathetic but actually I think that people who participate in a busy forum are entitled to do what your guys have done. If the forum stops offering them their platform they are entitled to look elsewhere.