First thing I would do is dial *228 then choose option 1 to reset your phone. Contacts, pictures and such will not be lost.
Make sure the sender and you are sending messages to the persons 10 digit phone number. Do not include the leading 1, and do not send using a 7 digit number.
If that does not work, I would call tech support from another phone so they can go through the settings with you.
Maybe texting was inadvertently blocked for some reason.