I'm not sure what would have caused the HDMI ports to suddenly stop working. If the devices aren't outputting at a high resolution - and check on the working port - see if you can bump up the output resolution (ie: cable/sat box, etc).
If that doesn't work, you might try calling 800-SAMSUNG to see if they can troubleshoot or set up a service ticket for you.
I have a pn50b650 for a week and I've been attaching two HD recievers to HDMI 1 and 2 with no issues. Last night I tried to connect a bluray to port 3 and it didn't work. Now none of the ports are working except port 2. All my Devices work fine when connected to HDMI port 2, but there's a NO SIGNAL message if I try to use the other ports. I've tried power cycling the tv and devices and it hasn't fixed anything. Anyone have any ideas?