I've done a bare-bones post-mortem and secured what I could. Looks like they got in via an administrator account that used the same password as on another compromised site. (Never reuse passwords between websites!)
Unfortunately, it seems plausible they got hold of a database backup through the compromised account. In terms of sensitive material, this includes things like private messages and forum-internal data like password hashes.
No one is going to recover any cleartext passwords from those hashes anytime soon, but there are some unfortunate design flaws in SMF (the software powering this and many other forums) that mean there are a few additional concerns. I've patched these forums to protect against these issues for the time being, but to be on the safe side you should change your password, as well as double-check that you're not using the same username & password combination on any other forums or website.
As part of general checkup and mitigation efforts, I've also enabled always-on HTTPS, wiped all existing sessions, erased any unused accounts and removed the ability to make database backups from administrator accounts.