make sure your plug is plugged in the same outlet from the last time someone/you hard powered it the lights were on and it had sound ,
if you have audio but not video, then the video inputs may not be plugged in properly to the dvr or computer,
but if thats not the case, theres a possibility that the hdmi cables are damaged inside and make sure your port isnt damaged,
put the hdmi in port 1... if that doesnt work try port 2 and port 3
and also make sure it is on the right hdmi setting,
if you have audio but not video then you know that at leat some of the chips and connecters on the motherboard of the tv are working... that will show the tv still has internal power...
if none of this works then you will need to open up the tv and if some of the compacitors are big and fluffy instead of their original compact shape it might be why the tv went dark.
if the tv is new then either take it back for service or replace it.
unplug it wait for 10 mins then plug it back in.... if its still dark after doing this power cycle then it could be the tecon board, the backline to the router or something else,
if none of what i have listed worked ..... you may be dealing with a damaged motherboard.
if this didnt resolve it i dont known what will