I'm not sure but maybe they "blocked" you and you can't send them texts anymore.
Or maybe they sent it themselves trying to tell you to stop sending them text messages. Heheh.
The main reason I wanted to answer is because I think calling and talking to someone is a better choice than sending text messages most of the time. Unless you're sending a quick address or something. I think it's more polite instead of typing out something for 20 minutes that you both could say in 5 minutes over the phone. Also for dating type situations when they call it feels like they're putting more respect into it. Somehow sending text messages became extremely popular. Hehe. I think talking helps decrease misunderstandings about tone of voice. I think people should get up the guts and talk to them.