Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
First we can reset your iPhone and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot.
Then go to Settings > General > Reset and tap Reset All Settings. This will reset some settings like WiFi, bluetooth, wallapper etc. But you will not lose any files or contents.
After resetting all settings, you will have to connect to your WiFi again from Settings > WiFi.
Try to send message now.
Please let me know if that didn't work, so we can continue troubleshooting.
If this doesn't work, please don't rate me bad or poor. Instead please reply back to me and I will continue to help you
Also turn off the phone. Take out the SIM card, wait 2 minutes and put it back. Turn it back on. See here --> https://support.apple.com/en-in/HT201337
Go to Settings > Messages and make sure iMessage is set to ON. If asked to activate iMessage, do it.
Let me know the results please
Are you able to receive messages ?
Are you able to receive messages ? Also did you try resetting network settings ?
If it still not working, then you have to restore a fresh software on your iPhone from iTunes on your computer. Follow the steps below;
First download and install latest iTunes on your computer from here : https://www.apple.com/itunes/download/
Then connect the iPhone to your computer with the USB Cable.
Open iTunes application on your computer.
iTunes will recognize your iPhone. Select your iPhone in the top left (select iPhone icon)
Then in Summary section, click Backup and make sure "To this computer" is selected.
Then click Restore button and let it restore. It will take some time to restore the iPhone .
After restoring, you will get option to restore your backup. Select Restore from iTunes backup and all your files will be restored. Try to send messages now.