Why am I banned from a certain channel?
There are two possibilities.
You upset one of the operators by breaking the channel rules and they banned you.
You can try ask them to unban you if you know his nick by typing /msg <nick> <message> replacing <nick> with his nickname and <message> with what you want to say to him. Politeness in this situation might mean the difference between remaining banned and being allowed back in. It's up to you.
The other possibility is that someone else from the same or similar IP address as you has abused the rules of the channel. It's unfortunate that you might have been banned because of someone else's doing, but in some cases it's the only way the operators can remove the troublesome users.