I unplug the HDMI cable (just one end) and then power off then unplug the set for about 10 minutes. I apply power, plug in the HDMI and re-test.
If that fails, the maker is called. Be sure to have them check firmware.
My Samsung LED Model 6300 just started having a strange issue. It is hooked up to the cable box with component cables. For as long as we have had cable, the TV source has been set to Component and everything worked fine. Then, one day last week, no signal through Component. When I switched the source to HDMI, it worked again. Then, just as suddenly as it happened, a few days later, no signal through HDMI -- and it was now back when I switch the source to Component. Just now, it has done it again -- back to HDMI. There is nothing hooked in to HDMI -- as I said, the connection is through component cables. Any ideas?