MY LG 55" tv freezes at start up.I purchased a mother board(Model 55UJ6540-UB). I installed the new mother board and still the TV will not start/turn on. Could the Power Supply be the problem?

Just replace your tv power board.
LG EAY62171201 EAX63330001 3PAGC10038A-R PSPL-L014A POWER SUPPLY UNIT https://www.abctay.com/lg-eay62171201-eax63330001-3pagc10038ar-pspll014a-power-supply-unit-p-1875.html

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

without testing then you might as well replace them all.

Check the mother board not faulty,common problem.

Are you referring to the DVR function of the TV, if so what country are you both in? I believe the US does not allow the DVR feature on the US model TV's (or didn't when I worked for LG), Canada does.
Check you both have the same software level, look in the support menu for the software revision.

Looks like a problem with mother board. Call LG and they will replace the mother board in around Rs 7000/-

check for bad capacitor in the power supply board and then check shorted component parts at the sustain board.

Sounds like it started "Protection Mode" because of a failed electronic part inside the TV. When this happens, the TV will keep turning itself OFF or will not turn ON at all. This keeps the other part inside from being damaged. Most likely the TV has a power supply related problem. It's possible that it maybe a bad capacitor or transformer on the power supply board. The common part to go bad for flat panel screens circuit board that powers the screen. Most manufactures call it a Backlight Inverter Board.
But, I can't say it's 100% the problem because I can't actually look at the tv over the internet. I can only make guesses with the symptoms.

You have a few choices:
1. You can again try leaving the TV unplugged for 30 - 45 minutes so the TV can discharge and reset itself.
2. If by chance the TV is still under warranty, go that route.
3. If you have some electronic experience, if you can find and purchase a Service Manual over the internet; It will help you diagnose the source of the problem and fix it by soldering in the new part. The Service Manual will give you circuit diagrams, schematics, test points, and part# lists.Usually, cost under $20. And most cases the new part cost less than $15 by searching on the internet with the part number.
4. Pay a TV shop to do the repairs. It maybe costly, but get an estimate of how much they will charge. That way you can decide if it worth repairing or replacing the whole TV.

I hope I shed some light on the subject for you.

Most likely the power supply board.

Lie the tv down on a bed and unscrew the back. Its only about 9 screws around the edge and look at the power board, on the left, see if theres any burn marks.

I have a brand new board I bought the other day for the same issue but before I replaced it, stupidly, I checked with LG and turns out mine is still under warranty so I'm stuck with a new board.

Let me know if you want it. I'll match any price you can find online.

not sure what you're referring to as a "main board", but heres the LCD Control Board for the TV

http://cgi.ebay.com/LG-42LC7D-UB-LCD-CONTROL-BOARD-6870C-0119A_W0QQitemZ150277759127QQcmdZViewItem