First, disabuse yourself of the notion that any password based on special characters, numbers or mixed case is more "secure" than any other. Unless the system does not allow special characters or forces letter case, the search space is the same (and is actually smaller if special characters are required. Length is what provides strength. (So for anyone cruising here and thinking of creating their own login system, or choosing one off the shelf, limiting password/passphrase length is wrong, wrong, wrong. So is forcing character choice. The only limit you should be placing on a password/passphrase is a minimum length.) And if by "encrypted" you mean you hashed it with something like MD5 (and anybody knows about that) you've really restricted the search space by forcing a single string length. Brute forcing just takes time.
It is far more likely, though, that you gave the username and password away yourself. I don't mean that you did it deliberately, but if you used a regular HTTP connection over open (unencrypted) wifi, like a public hotspot, the information was out there for anyone interested to read. Anyone within radio range running software like Firesheep can get your credentials if you use HTTP over an unencrypted network.
I wouldn't worry too much about Anonymous (the group) unless you're an "evil" corporation, a government agency or Scientology. It's very unlikely that a small, private site would interest them at all, and defacing hobby sites is not what they're about. It's more than an even chance that you were the victim of a "script kiddie" out for a bit of fun.
As for what you can do, well, if you can still log into your account portal (from a secure network this time), you can change all of your passwords for that account at one go. If you can't, but can still log into your forum account, you can request that your account be terminated immediately. If you can't do either, you should be able to use the "forgot password" function using your email address. Otherwise, you'll just need to let it go—requests need to come from your forum account in order to be verifiable (otherwise anybody could cancel anybody else's account at any time), and it's doubtful that your hacker will bother with the monthly logins required to keep the hosting account going.