imessage sign in failure

My ipad mini 4 started displaying an error message for imessages that says: sign in with your apple ID to activate imessage. When I input my apple id, the wait symbol just twirls and then never signs in. A new message appears that says : unable to contact the imessage server.

I have rebooted to no effect. I have been using imessage on my ipad for more than 5 years and have never had an issue.