Q: Imessage

I'm having triuble with my imessage. I am able to use my imessage for everyone in my contacts who has an iPhone but this one person. It was working fine on Sunday and yesterday just stopped.  The person I'm trying to message is having the same issue where it will work for other people but not me. I've turned off my phone, turned off my imessage, signed out of imessage and resigned in, and i just don't know what else to do. can anyone help?