There are different types of bans and the resolution of those bans vary depending on the type of ban.
There are channel bans, which are set by the owner and/or ops of the channel. If this is the style of ban, which you can check by connecting to the same network, but not being able to join the specific channel then you need to contact an op in the channel and ask them if they will explain why you've been banned.
You can usually determine the ops in a channel by typing
where #channel is the channel name.
If you are unable to stay connected to the network, due to the ban (properly called a line, of which there are different kinds), then you need to contact the ops of the network (called IRCops). You usually do this by going to the networks website, and following the directions given there.
The directions given vary from site to site and therefore, network to network.
