You’ll have to create a new API Key in MailChimp and copy/paste it into TntConnect, replacing your old/revoked one.
To do this, pull up "Sync with MailChimp" and click "Options".
You'll see a link to "Where is my API Key".
You need to create a new API key. Click "API Dashboard":
Then click the button to "Create a Key":
Then copy /paste that new API key into TntConnect over top of the old one.