Are you sure mIRC doesn't react to any of your commands? Or is it the IRC server that isn't reacting. If you can connect to the server, but can't do anything on it (ie. join channels, send messages, etc) then you are likely shunned. A shun is like a ban that prevents the IRC server from responding to any of the commands you send to it. If I recall correctly, only the /admin and /quit commands work when you are shunned.
And before you say "I didn't do anything to get shunned", know that shuns can be wide like glines, so someone with a similar hostmask to yours could have gotten the whole range shunned.
-genius_at_work
