You've got probably shadow-banned, meaning banned from chats and forums. Maybe you insulted someone in a game and your opponrnt reported you and so the mods banned you from chats.
You can find more about it here:lichess.org/qa/184/is-ban-in-the-chatforum-forever
Or you simply had the chat turned off. By clicking the tick-box next to where there is written "chat room" you can turn it on again. Probably it is because of this and not because you've been banned ;)