My iMessage isn’t working

See also

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

 and

 Problem activating iMessage

See If you can't turn on or sign in to iMessage or FaceTime – Apple Support (UK)

 If you recently changed phones

Deregister old phone iMessage online

If you don't have access to your iPhone, you can  Deregister and Turn Off iMessage - Apple Support

Then contact Apple Support for further investigation. You should be able to reach them here

Contact - Official Apple Support

or phone numbers

Contact Apple for support and service - Apple Support

Set up imessage and facetime

Settings, Messages, Turn off iMessage

Settings, Face Time, Turn off Face Time

Restart the phone.

Settings, Messages, Turn on iMessage, SMS, MMS

Settings, Messages, send & Receive, select your number which should now be there.

Settings, Face Time, Turn on Face Time