Samething happened to me, in short the digitizer is damaged. You have to replace the LCD/Digitizer which are combined. Better send it to LG they will do it for between $140-150 after taxes. Unless you can replace it yourself, I believe the part is about $100.
(Stock ZVC Rom)
Past Devices: HTC Butterfly, HTC One X - Tegra 3, LG Nitro HD, LG G2x, Samsung Vibrant, Samsung Galaxy 3, Nokia 5800, Motorola Razr Maxx, Palm Treo 650