Text/iMessage Reversal Help!!

Hey Everyone! Over the weekend I was trying to send an imessage and it wasn’t going through so, my phone asked if I wanted to send it as a text message. I hit yes. I've done this multiple times on many different occasions and have never had a problem. Now, my phone will only send messages to this specific contact as text messages and not iMessages (when they are able to send iMessages to me). I have deleted the contact and added it again, deleted all texts/imessage conversations, turned imessage on/off and nothing works. How do I fix this problem?