Go to Settings > Messages > Send & Receive and make sure that you have registered iMessage with your phone number or Apple ID and that you have selected iMessage for use.
Next, reset your network settings. Go to Settings > General > Reset > Reset Network Settings. You need to have your WiFi password available as this clears out all stored WiFi and network passcodes.
Sign out and back in with your Apple ID (GO to Settings)
Toggle iMessage OFF and then back ON (Go to Settings)