The EMC servers are down. This message is displayed when logging in, "you can only connect to one EMC server at a time." The EMC server are offline more often on weekends nowadays, why?.
What's likely happened is that you lost connection on one server, and then tried to join another, but you're still logged in on the other. Try joining the same server you were on before.
The servers are up, it just means that your player is frozen in another server. Give it a hour or two and you should be able to join again. Or find the server that you were last on, and hope that it lets you on.
The EMC website says SMP2 is online, but my client says otherwise, and I keep getting 'Connection Refused: Connect'
Someone changed something, because Chickens post once said what IcC's says... >.> I sense a conspiracy!