UPDATE: It works once again!
The forum is defaulting to WSOD (White Screen of Death), which usually indicates some sort of EBMU (Error Beyond My Understanding.) By systematically renaming all the files through FTP, I’ve traced it to an htacess file. Unfortunately, renaming htacess file restores the forum, but nukes the rest of the site.
So. I’ve emailed the host and asked them for their help. We’ll see what happens.
PS. The forum breaks like clockwork every six months. Argh.