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It's possible that the backlight is the only problem with it - if the times you *can* see anything on the screen, are when direct light is shining right into it. That makes any LCD-type display with a dead backlight, slightly usable.
At very least, the backlight connection has worked loose. At worst, the whole screen needs replacing at minimum - if the sort of screen in your LG Optimus' particular exact model, is the type where the backlight is built into the screen module.
(Let me know your Optimus' exact model #, to hopefully get a more exact estimate as to whether the backlight, if broken, might be separate and replaceable without replacing the screen too.... that would reduce prospective repair cost.)
I advise you to use what little screen readability is left, before something gets worse, to get all your personal data that is stored on the phone - pictures, video, contacts or Address Book, notes or documents - copied off it onto a different device e.g. a computer (or at least a memory card if possible.)