I suspect if a member was banned from a site running forum software - be it SMF or Vbulletin I suspect when the account was accessed a message would deny login and state 'you have been banned' or email admin re your account being locked.

Admins periodically do clean outs of dormant accounts - if one has not been accessed or if there is multiple accounts from a single I.P etc.

If you are receiving a server error, it could be a couple of things - ie your account has corrupted or a database problem on the server end.

I would suggest to delete all temporary internet files and cookies and try again.

Definately email the forum administrators as well - primarily regarding your account (and for the security of their site) if there are issues with the backend user database or it may simply be a software issue.