Wrong place to make a post about this issue, it should be posted in the MiningDead section.
To solve it, have you tried relogging ? If it still doesn't work, try logging in and out of your account.
For the last solution: Go to options --> Chat Settings --> Chat: Shown as mentioned by Fox above.
If nothing of the above works, do ./report in game and submit a ticket regarding your issue.
Anyways, G'luck !
-Wavy