This is Twitch’s third party developer forum. Issues pertaining to developers should only be posted.
With that said, your issue could be that a moderator has banned or timed you out from that particular channel. This could be easily identifiable because there would be indications telling that you cannot message the channel. That however is not permanent because each channel has the right to ban or timeout you out so if you are able to talk in other channels and they can see your messages this is most likely your problem and should be take it up with the channel owner and/or moderators so they can unban you if they so choose.
If you need additional help please refer to the help page
If you cannot type in any channel no matter where you go then this is most likely a bug and should be reported by submitting a report for further assistance.