Perform a series of troubleshooting procedures to diagnose and fix an iPhone that can't send SMS messages. After each troubleshooting procedure, attempt to send an SMS message. If you can't send a message, proceed to the next potential solution. If none of these solutions remedies the problem, contact your mobile service provider. You may have a problem with your SIM card or with some element of your iPhone's hardware.
Restart your iPhone.
Make sure you're connected to a cellular network. Open the Settings app, tap Cellular and toggle the Cellular Data switch off and then on again to reestablish its connection.
Disable and reenable iMessage, if it's the messaging app that's giving you problems. Open the Settings app, touch Messages on the main settings menu and then toggle the iMessage switch off and then on again.
Force-Quit Your Messaging App
Restart your messaging app in case a technical problem has developed with the app itself. You can quit an app normally, but it sometimes continues running in the background. Force-quit the app to make sure it restarts.
Double-tap your iPhone's Home button to launch the multitasking screen, on which you'll see a list of all of the applications currently running on your device.
Swipe through the thumbnails until you locate your messaging app, and then swipe its thumbnail toward the top of the screen to force-quit the app.
Restart your messaging app and try to send an SMS message again.
Reset Your Network Settings
Launch the Settings app and touch General to view the General settings panel.
Touch Reset and then tap Reset Network Settings to return the iPhone to its default networking configuration.
Perform a Hard Reset
Press and hold down both the Power/Sleep button and the Home button on the iPhone until the Apple logo displays on the screen.
Release both the Power/Sleep button and the Home button and wait for the iPhone to restart.
Install the Latest Software Updates
Occasionally apps and the iOS operating system itself develops bugs that can be fixed only by installing software and firmware updates directly from Apple. Use the Software Update feature on your iPhone to install these free upgrades.
Connect your iPhone to its charger to ensure it has a constant supply of power throughout the update process.
Launch the Settings app, touch General and choose Software Update.
Touch Download and Install and tap Install to install all available updates for your iPhone.