If the network you are trying to get on is DALnet read this
. You are better off using a different server until the attacks stop (if they ever do). From the servers list select a different server (Try Quakenet, IRCnet or EFnet). If you can't get on any of those post here with the exact error message.
If you are not trying to connect to DALnet then the server might be temporarily down, wait a while and try again.