You did a nand backup before flashing Megatron, right? If so, you should boot in recovery using Home + Vol Down + Power and restore it from there. If not, you have no alternative but flashing the rom again (or any other rom). Be sure to wipe everything from recovery before that.
If that fails, you should get the official rom for your location and flash it with kdz (there is a very well done guide here on XDA for doing that, search for it).
And consider flashing Void next time, is more stable and has the same features (and more) as Megatron (hwacc, jit, oc, stagefright etc).
In any case, if the phone boots, even if gets stuck on LG splash screen, it is not bricked. It can be re-flashed easily, just follow the steps in the guides.
How to flash original rom:
http://xdaforums.com/showthread.php?t=875881
Original rom list:
http://xdaforums.com/showthread.php?t=865479&highlight=stock+rom