My supersonic led tv showing dark picture and no sound

Depends on the model. Have you checked the backlight, (LED in this set perhaps).

The backlight led is burnt out, time to replace with new one. Most of the time it’s only 1-2 led are burnt. You need an electronic technician though. I may guide you if you want further assistance.

Sounds like a failed led or led backlight. Some googling will point you towards replacement parts.

Was the set dead before you replaced the power board?

thanks,
ding

If it shows only a blank white screen, means, there is no video signal. Just try it with external Ariel and see whether there is picture tunes in. If not, the main board to this set is likely faulty. Contact any service technician. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Pull up older posts. Surf the site with patience. Do a search by type in the brand name of your device at its search box.
http://electronicshelponline.blogspot.com/