Steps to restart iPhone to fix message send failure issue
Below are the steps to restart your iPhone in order to fix your message failed to send iphone issue:-
1. If you are using iPhone 6s or earlier then simultaneously press and hold the Home + Power buttons for 10–15 seconds and release after the device starts to vibrate.
2. If you are iPhone 7 or 7 Plus then press and hold the volume + power button at the same time.
3. If you are using iPhone 8 and later models then quickly press the volume up button, and then rapidly press the volume down button.