In the past, when a guest hasn't responded to my Airbnb messages, SMS texts or phone calls I've contacted Airbnb asking them to see if they can make contact. In each case the guest has contacted me or I've received a message from Airbnb saying that they've contacted the guest and giving the guest's response to my question.
For information on ways to contact Airbnb support type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
@Neva10, I've noticed that a lot of people, especially if they've just signed up on Airbnb seem not to have notifications turned on. I have had a few instances, where I have not heard anything back from a guest, so I've had to end up texting the person. In both cases, they have responded at once and were surprised I had been trying to get in touch with them.
I have recently entered a line on my IB message and on my listing asking guests to turn on notifications so we're able to communicate.