by AverageJoe » Fri Jul 10, 2015 4:41 pm
/leave the chat channel the /join it again.
If that doesn't work, navigate to your World of Warcraft Classic\WTF\Account\%account name%\Nostalrius Begins\%character name% folder and delete the chat-config.txt file. This file stores all your chat window settings. You'll lose any custom configurations, but it'll get recreated the next time you login and can reconfigure it.