Pleade help ? My imessage won't work with only one person?

My imessages won't work with only one of my contacts and neither will facetime . They are not blocked . I have tried turning imessage on and off , facetime on and off , restarting my phone , and deleting their contact then re making it , and sending them a message through imessage from my email. How do I fix this ??? It's on my end not theirs . Please help!!!!!! We have always been able to text just fine this started a month ago out of nowhere .