[Shodo]

As mentioned, make a post in the petition section (and select the bans/suspensions tag).
As for possible reasons for the ban, the following are all bannable/suspendable offenses as far as I know:
*Real-money trading (a.k.a. RMT), or exchanging any form of currency outside of the game for platinum/items/etc in-game.
*Multi-boxing, or playing on more than one account simultaneously on the same server (one character on blue and one on red is fine, last I heard).
*Using 3rd-party programs etc that either automate gameplay or add additional functionality beyond what is possible by simply reading the log file (some log-reader programs have been allowed so far, but they're probably reviewed on a case-by-case basis).
*Not sure on the exact criteria, but I believe bad behavior can result in at least a temporary ban as well (griefing other players via repeatedly training or kill-stealing would probably at least get you suspended, for example).

In short, after posting your petition I would go read through the server rules again incase you missed something.