Error message on iPad 5 - iMessage Activation Failed. No problem with iphone or macs

The full error message states: iMessage Activation Failed. An error occurred while activating iMessage. Please contact Apple Support to resolve this issue. The options on error message are (1) Cancel and (2) Contact Apple Support. Tapping the cancel button does nothing. Tapping the support button opens a link to Apple Support via Safari.

I cannot access iMessage settings because the error message window prevents menu access.

My iMessage account was reset several days ago by apple support. Prior to this my messages were okay on all my linked devices. I checked that iMessages on iPhone and Macs were working after action by Support, but did not immediately note the iPad had failed to restore.