When your TV has a black screen even when it's on, it's quite possible it could actually be your external device and not your TV. If your TV is connected using a One Connect Box, there are a couple of extra steps to troubleshoot.
·Press the Home (Menu) button on the remote.
Confirm the TV is set to the same source as the external device.
Test the HDMI cable.
Test different external devices.
If there is still no picture, use the following link to find a list of service centers near that can do the in/out of warranty repair. http://www.samsung.com/us/support/service/location