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 iPhone as iOS clears cached data and does other system optimizations when you turn your device off and on.
Restart your iPhone, iPad, or iPod touch
Force an app to close on your iPhone, iPad, or iPod touch
If you're still having this issue, our next step is to have you delete this thread and start a new thread. Before we do this, please backup your iPhone and if you want to save parts of a message, take a screenshot . To save an attachment in a message, touch and hold the attachment, tap More, then tap Save.
If you can't send or receive messages on your iPhone, iPad, or iPod touch
How to back up your iPhone, iPad, and iPod touch
Import photos and videos from your iPhone, iPad, or iPod touch