I had a similar issue. I had been receiving messages and suddenly, I couldn't receive one person's text messages. I didn't want to reset everything. I thought there had to be an easier solution. After backing up my address book (just in case it all got deleted), I went to iTunes and updated my iPhone software. This made me feel better, but didn't solve the problem. After further investigation, it turns out that my iMessage was turned off. The other party was sending from iMessage instead of SMS to text. To fix: I went to "Setting", "Messages" and switch iMessage to "ON." My problem was solved after that step. I hope this helps others.
(Moved this post to a new discussion for better exposure)