Have you checked any other servers on that network and/or other networks to see if you have the same problem?
If it's just that server, then use a different server. If it's the entire network, go to another network. I know this last one isn't always possible, as sometimes things are arranged to happen on a specific network, but if you can't send any commands to the network, then it's useless being on there.
Have you tried using /debug @debug to view the commands sent and received between your computer and the server? If so, what are you seeing for things like /join <#channel> or /list?
