First perform a factory reset if possible. Then, connect a simlple DVD player, etc. to the HDTV and see if that works. If so, it's likely another issue, potentially with your laptop. Sometimes getting the video output settings just right can take a little futzing IME.
I opened up my new TV today, and after discovering that my coax jack in the wall doesn't work (i.e. I didn't receive any chanells) I decided to see if I could watch videos from my laptop (2007 asus running XP). I couldn't get that to work (the screen was black), so I unplugged the D-sub cable from the TV. When I tried to switch back to TV mode, nothing happened. Now the only button that does anything is the power button.
I have tried everything on the support site, including contacting customer service (who, as usual, told me to repeat all of the things I had already done), who ended up telling me that I need to have it serviced. I've literally had this TV for 6 hours, is there really no other option?