Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to send messages to a specific contact in the Messages app on your iPhone. If you haven't already, please force close the Messages app and restart your device as iOS clears cached data and does other system optimizations when you turn your device off and on. Also, check with this contact that they didn't accidentally block you on their device.
Force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone, iPad, or iPod touch
Block phone numbers and contacts on your iPhone, iPad or iPod Touch