I have uploaded and tested Marlin Firmware with full graphic LCD and it was fine. But, when Ramps is assembled with stepper drivers on, LCD suddenly powers off. My PS can output 12V 30A. Can steppers drivers got burned?

The LCD runs on the Arduino 5v regulator, so either it is weak, or something is compromising it. The stepper drivers use that 5v to derive Vref, which by itself pulls almost nothing, but if one is bad or plugged in backwards, it could apply a load. Auto level servos, or miswired endstops can also dork the 5v supply . . .

- Tim