Turning off and on iMessage can also assist with small technical issues like this. FaceTime must also be turned off in this situation. Here's how you do it:

• Go to the Settings app on your iPhone.
• Select Messages.
• iMessage should be turned off.
• FaceTime should be turned off. (This is a must-do if FaceTime isn't working.)
• Your iPhone should be restarted.
• Return to Settings after the reset and toggle both iMessage and FaceTime back on.

Check to see if your phone number is in your contacts.

For iMessage to operate correctly, your phone number must be in your Contacts list. Open your contacts and check to see whether you can see your own phone number in the upper left corner. It's usually branded My Number. If your phone number isn't listed, go to Settings-> Phone-> My Number and input your phone number.

Make sure the date and time are right.

Check that the date and time settings are correct. Firmware upgrades can occasionally mess with your phone's settings, forcing you to adjust them afterward. As a result, double-check your iPhone's date and time display to ensure that both the date and time are right. In the General settings, you may also set the date and time to 'Automatically.'