I need to replace LG screen 55 inch - LG Televison & Video

no actually not i think you rather shift your choice

Sounds like an issue with the motherboard or a short in the screen circuits.
As it usually costs more to replace this than most sets are worth I suggest purchase a new one.

Main digital board fault. Check and replace it. If you wish to get more details visit the site linked here. http://electronicshelponline.blogspot.com/

Try shopjimmy
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience.
http://electro-medical.blogspot.com/

Back - light problem. 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://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.

it is for US$ 1,268.90 part only...plus labour etc. etc.
EAJ62111301 Part LG Electronics Zenith EAJ62111301 LG Electronics Zenith...

If you hit the studs I would use a 7/16th lag screw at 2 1/2 inches. Not to make this sound like I am guessing, I put these up all the time for work. The lags should be included with the mount though.

It will cost more then the cost of a new TV to replace that LCD panel. 55" panel would probably be in the $700-$800 range. Walmart has new 55" LCD's in the $700 range. The TV is not junk, it has some value. Throw it on Ebay or Craigslist as a parts unit. I assure you someone will buy it. I'd ask for at least $100