You dont say if you were banned from the server/network or banned from a channel. You can be banned from a channel for any reason, up to and including none at all. If you were banned from the server, there was a reason. Bans can be permanent or of any duration, from minutes to whenever. You may be able to find who to talk to about your specific problem on EFnet's forum
You can see general rules when you join a server by reading their MOTD (Message of The Day), which you see when you join or by typing /MOTD There are some rules which are pretty much a given, like no flooding, no mass advertising, etc. Check out this site
for some good general info for new users who want to use mIRC for its intended purpose, chatting.