Open Settings
Tap Messages
Turn on iMessage
If prompted, sign in with your Apple ID
Make sure your device is connected to Wi-Fi or cellular data
Check that your date and time are set correctly
Restart your iPhone if iMessage does not activate
Go to Settings
Tap your name
Tap Sign Out
Sign back in to your Apple ID
Go to Settings
Tap General
Tap Transfer or Reset iPhone
Tap Reset
Tap Reset Network Settings if iMessage still will not turn on
Contact Apple Support if iMessage remains unavailable