What can I do to change the contact I'm texting, back to iMessage?

I have an iPhone 5c. I have been using iMessage but I was texting someone and my iMessages wouldn't send because I wasn't using Wi-Fi at the time and my 4G was turned off. I simply pressed "send as text message" every time I was sending an iMessage. It automatically started changing to text messaging every time. It would go back to iMessage sometimes but I would keep pressing "send as text message" and now my phone won't go back to iMessage with that certain person. I have turned my phone off and even restarted it.

The problem may be at their end, if they aren't correctly logged into iMessage.

Imessage works on Wi-Fi or celltower.