Pics, or it did not happen. Jk.
Unless I am mistaken, I believe LG is required to honor their warranty despite how many hands the item has changed. If, for example, the TV was warranted for 3 years, and you purchased it from your friend at the beginning of year, you still have one year. An express warranty does not terminate when ownership changes. An implied warranty of merchantibility, may be the type of warranty difficult to argue for, as it requires proof that the TV was in a unusable/broken state when it left the manufacturer's possession. **Edit** - RandyWalters has shown that LG's express warranty excludes second-hand purchasers. -- see below.
I have been warned twice not to turn my TV on when it was in a cold state (e.g., was shipped in Chicago). Further, my plasma gets quite hot when turned on. Given the extreme temperature differences, I could see the glass cracking. Is your TV next to a poorly insulated window or seam?