Sound works, picture is very dark

usually due to loss of back lighting

HI there do check all circuit boards in back cover for any blown fuses or other electronic components , If all good then you will have to remove LCD screen to test LED lights . switch TV on from standby mode and shine a LED torch onto screen with room completely dark if ICONS do display ,Then i wood recommend for you start off checking your power supply circuit board or driver board . A easy way of checking if LED lights are working again in a dark room with TV switched on look through back cover vent holes for any light shinning through small openings this would determine if some or all back LED lights are blown.

You need to replace the screen back light.

Darken the room. Very dark. Turn tv on. Press menu button. Get flashlight and shine at screen. If you can make out the menu barely. It's the back light gone. Is it plasma or lcd.

Hi,
it sounds as though some of your LED back lights are not running, you can verify this by shining a flashlight onto the dark area of the screen (in a darkend room)to see if there is a picture there but not back lit. If this seems to be the case it will most likely be the back light inverter/driver board, on some sets this function is carried out on the power supply board, have a look at shopjimmy for tech info on your brand:
TV Parts Universal TV Stands DLP Chips ShopJimmy

Check and replace its back-light inverter board. Contact any service technician. Gopakumar Gopalan Google

Sounds like you have a backlight problem. To verify this, turn on the set and dim the lights in the room. Take a bright flashlight and shine it on the screen. If you see a faint picture, you either have a back inverter or a bad backlight bulb.

Show a torch light on the screen for a distance, and if you can see the picture, the backlight is not lighting up. Check for its settings on the remote, if you have that option. Check also the settings of brightness/contrast/sharpness/color. If you still have problems, please call in a qualified service technician.

The Inverter is bad - this is used to power the back-light of the screen. With a flashlight, shine the light on the screen to see if you can detect images. If you do, then that confirms a bad Inverter or the back-light bulb itself bad.

Shine a small bright light at the screen when on and see if you can see a faint picture in the dark screen. If you can then your backlighting is not functioning due to ccfl (inverter), power supply, or back light lamps.