I have a LG 50LN5100 UB with sound and no picture. I hold a flashlight to screen and can see words with the flashlight. What parts should I replace to get the tv working again

You have bad LED backlight strips, VERY common failure, it amazes me how many people change the power board with no backlights and in most cases it's not the power supply that's bad

Probably a failing power supply

The led driver AND the T-con board are bad. If you could see a picture with the flashlight then the T-con board would be good. It is not. The led driver can be on the main board or the power supply board. You can get these parts from here: TV Parts Universal TV Stands DLP Chips ShopJimmy

You have lost your backlights and this is very common for the LN series LG TVs...they use 6 volt lamps and if one opens the entire string goes out, kind of like the old Christmas lights used to be. First thing to check though if your able is the voltage on both sets of cables, hook your meter to one side of the power board that supplies the lamps and see if you have voltage for a second or 2 when you power the set up and then it drops off to almost nothing, if you see this unhook the plug that goes to the lamps in the panel and see if the voltage holds at a couple hundred volts, if it does its lamps in the panel and I think you will find this is the case, if it happens the voltage drops down or isn't any to start with its the power board...Good luck

The backlight

Defective LED back lights strips.

Check out the links below to eliminate the Inverter circuit
and / or LVDS board
Instruction for LT30FMP

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://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.

Hi Lisa,
Both picture and sound missing indicate a bad main board.

-You have a bad panel. Led arrays internal to the panel are defective. Replacement cost exceeds economical repair.