That message means you need to be invited to the channel by an op (@) to enter it. This is done by a channel 'mode' called +i (invite only).
If you know an op's nickname then you can message one and ask to be invited in. Use /msg nickname messageHERE. If not, you're probably not welcome to the channel, maybe the channel has decided to set +i from now and changed it's policy to let people in.
Sometimes channels will set +i automatically (by scripts or bots) when they get a flood. Occasionally this will mode be forgotten to get unset - so wait a few hours and see if you can get back in. Usually an active op will wake up and notice that the mode is set and unset it, so it might only be a temporary thing.
Also check the channel's topic, type /topic #channel. It might have instructions on how to get into the channel (i.e. message a bot).
Hope you get back in soon :-)