Check that the number you are trying to send the text message to is correct.
Check your signal / reception. (Click here for more information)
Check that the person you are trying to send the text message to is not overseas, that their phone isn't broken or that there is no other reason they cannot receive messages.
If you're having problems receiving text messages, your Inbox might be full. Deleting old messages might fix the problem. To delete messages one by one select the message you wish to delete, choose Options and choose Delete. You can also delete a group of messages by choosing Messages on your phone, followed by Delete Messages.