Samsung LCD model ln40b500p3fxza stopped turing on. No standby light. Replaced power supply board part BN44-00264B TV still does not power on. What else would cause it not to power on??

Try a power reset. But if you try to fix, this is the likely outcome. It is worth fixing, but do it properly.

https://www.fixya.com/support/r29790007-repairing_own_television

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The standby light means the tv is plug into the wall and getting power. A power surge can wipe out a tv and still leave the power light on.

Hello ? Is there any sound?

SAMSUNG BN44-00203A BN44-00202A IP-271135A SIP468A POWER SUPPLY https://www.abctay.com/wholesale-samsung-bn4400203a-bn4400202a-ip271135a-sip468a-power-supply-used-p-708.html

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. 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 digital board./

I didn't pull up the manual on this set but if since its a LCD set and the power board didn't fix it I'd look to the inverter board unless its built into the PS

Suspect power supply board.
Look for some poped capacitors or other components like fuse/power transistors

Repair No Power Power Supply Common Symptoms Solutions How to...

and

Fixing the Blown Capacitor in Samsung TV Yourself LCD TV

You can also replace the bad capacitors on the power supply board.

set has a shutdown or failsafe circuit:

I see so many here with no sound/nopicture issues:

ISSUES:

1. IF sound main fuse/s are good.
2. If no power or standby check main fuse, standby power supply and any signsof damage you can see---burnt parts etc.
3. almost all these sets the troubleshooting involves checking voltages on the main power board to narrow down theproblem.
4. Be safe and if you do not know what you are doing don't open the set.

Answers to issues:

1. RE:fuses---if a blown fuse is found get a good replacement(not the genericones like Radio Shack). Many fuses are special even aside from amperage--Don'tput a higher rated amperage fuse in the set for any reason.

2. RE: power supply in set: if total dead aside from main fuse the set uses astandby power supply that runs 7/24 and a main supply (some are combined andsome not.

Here aside from checking fuses you want to examine all the filter caps on thepower/standby to see if any are puffed up or buldged up in some fashion--Iattach a photo of a good one---top should be silver metal and PERFECTLY flatand have no brown material coming out on top or bottom.

What happens in the standby area is they use caps with too low a voltage rating(saves money) which causes them to fail in a year or two---the standby runs7/24 and the heat dries out the caps and they fail.

3. Trouble shooting consists of the above and then at some point you have tobuy and try a board to see if it fixes it or not.

4. Safety first--no TV set is worth getting injured or even killed over.