Unable to text a contact number that is green

I have never had an issue texting people in my contacts before but recently when I try and text a contact in my address book I get a return message that says the message was not delivered because I was trying to send to a non-wireless number. I don't know when this changed or Wyatt changed, but I've had some of these contacts for years. I see other people who have had the same problem and it's not been addressed very well so what happened? When did it happen and why can't I text people that I've been texting for years?