Sound is OK. Why is screen black?

either the power supply is going out or the led strips are failing they call them back lights

replace the led lamp inverter board

get it fix in local repair shop

verify back light using the flash test
then tcon
then the lcd panel

TV Repair Tips ShopJimmy
LCD LED TV Repair No Image Blank Black Screen Flashlight Test How to Fix... LED TV Repair Tutorial Common Symptoms Solutions How to Repair LED TVs

Perform a Flash Light test to confirm you panel and inverter states. LCD LED TV Repair No Image Blank Black Screen Flashlight Test How to Fix...

Eliminate the Back light, Inverter, Panel itself for defects

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

LG LED TV has no picture (black screen) but still sound ...

www.tomsguide.com/answers/.../led-picture-black-screen-sound-fixed.ht...

7 days ago - My 32' LG LED TV has suddenly lost picture! The sound is all right but theScreen is pitch black. After switching it on the picture flashes for a second and thenthe ... LG LED TV has no picture (black screen) but still sound - can it be fixed? Tags: ... a touch on the screen reveals nothing so it's not the back light.

Monitor going black after 2 seconds [Solved] - CCM.net

ccm.net > Forum > Hardware

Oct 9, 2012 - Brightness goes down, display draws less juice, and you can at least use .... it's the LCD's Inverter ... you have to replace it for it to go back to ... If the screen is turning off every two seconds or so and then it comes ... i turn my tv on it comes right on, but then after a few minutes turns off, .... Digital Trends for LG. No display in monitor
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TV with Black Blank Screen Repair How to Troubleshoot ...

? 3:16
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back light inverter board dry soldering OR CCFL damaged

I had the same exact problem.
I removed the bulb and cleaned it. There is a small screen at the top of the bulb and it was covered with dust. Once it was cleaned I replaced the bulb and it solved the problem.
Thanks to those who posted the solutions