One common solution to this problem is to unplug the power cord to the TV set and leave the set unplugged for about an hour in order to reset the TV's internal memory.
Then plug the set back in and try using the LG default Parental Lock passcode of 0-0-0-0 if necessary.
Here is a link to the LG Support site where you can download the User's Manual for your set:
Just use your mouse to click on the link, which is the underlined and highlighted text just above. This will open a new web browser page automatically for you and allow you to view the information and items at the website indicated.
When you get to the above webpage, just click on the TELEVISION MANUAL - ENGLISH button on the right hand side of the page.
Then select OPEN.
When the download completes in a few seconds, you will be able to view the contents of the manual. See page 99 of the manual for details on how to set and change the Parental Lock feature.
You may need ADOBE READER installed on your computer to read PDF documents.
You can download this free at WWW.ADOBE.COM
Click on the DOWNLOAD ADOBE READER button on right side of the ADOBE home page when you get there, then follow the onscreen instructions to install the software on your computer.
Please comment back here to this page if you have any details to add, have any questions, have difficulty accessing the links, or just require further general assistance and I will respond as soon as I see it.
I hope that was helpful to you, good luck and take care.