NO PC = NO LCD as the LCD and ramps are powered by the PC. One of the functions of USB is to provide power. I had the same issue, I am sure the backlight of the lcd is lit but no text is present when the PC is unplugged. This means the ramps board is not getting power, but the LCD backlight is powered by the ramps and not by the mega. I managed to fry the voltage regulator on my ramps when I shorted vcc to 12v vs vcc to ground. If you search end stop issues you see what I mean. Or you can head over to my blog and look at the heading "failure is always an option"
To me it sounds like you either fried the Mega voltage regulator or it was a dud to begin with. There are ways of bypassing this or repairing it as well.
To me it sounds like you either fried the Mega voltage regulator or it was a dud to begin with. There are ways of bypassing this or repairing it as well.