I just got a PN50C8000 and have a similar problem. When I first started the unit it synced with my Comcast HD box with no problem. I turned the set on and off a few times to confirm the connection and I then ran it for about eight hours. On turning it on the next day I got the "No Signal" response. I tried connecting the Comcast box directly, and to the sets other HDMI ports and got the same result.
Now, my normal connection chain is having all my sources plugged into a simple 4x1 HDMI switch that is feeding the set. What I'm doing to get the set to sync with the Comcast box is to turn it on while switched as a second monitor to my Mac Mini. I can then turn on the Comcast box and then switch the set to it.
I'm not sure why the set is not syncing to the Comcast box, but I'm starting to think its the Comcast box.