Ramps 1.4, Arduino Mega

I recently built a Prusa i3 3D printer. It worked for a while, then one day i tried to plug in the USB and the kill alarm from the ramps SD Card reader went off. Now, the Arduino will not power on when i try to power it with the external power supply through Ramps. however the arduino works fine when I try to power it through USB, but the motors for the printer wont work. The Ramps sheild is outputting power to the arduino Vin and ground pins, but the arduino is not powering up. Any suggestions on how i can power it?

ghershewe:
The Ramps sheild is outputting power to the arduino Vin and ground pins, but the arduino is not powering up. Any suggestions on how i can power it?

I assume you mean 12V on Vin. That should work unless the 5V regulator has melted. The stepper drivers get VCC from the Arduino +5V pin so they won't work if the regulator has fried.
The best solution is to get a 5V DC-to-DC Buck Converter. This is an efficient way to convert 12V DC to 5V DC. Feed that to the +5V pin and you can power both the Arduino and the RAMPS board from it. 1A should be sufficient because that is the limit on the built-in regulator.
This one should work: Mini DC/DC Step-Down (Buck) Converter - 5V @ 1A output [TSR12450] : ID 1065 : $14.95 : Adafruit Industries, Unique & fun DIY electronics and kits