I have a UN55C6500 LED and I have the exact same problem:
TV wire connected to a cable modem (ISP = Comcast), all network test passed. I got all Yahoo widget, but press "Internet@TV" button on the remote control, get to the Internet@TV main screen, but everything is disabled, except the information box on the middle left. I can not get to setting or Samsung app. I chatted with Samsung tech support online, I was told to "reset" TV to factory default setting. I was afraid to loss all my settings (which took me a few days to get where I am now). Wondering if anyone here has tried "reset" TV, is there any luck?
BTW, I was also given the "reset" steps as:
To perform a Factory Reset, follow the instructions below:
1.With the TV on, press and hold [Exit] on the TV remote for a longer time for 10 seconds
2.A warning message appears on screen. Understand that all of your settings are lost upon completion of the Factory Reset
3.Press the [Left Arrow] to highlight OK, and then press [Enter].
After the Factory Reset is completed, the TV powers itself off. Upon powering the TV on for the first time after the Factory Reset, the Plug & Play screen appears just as if you had turned the TV on for the first time.