You most likely burnt out your D2 diode.
Most people wire the 12V backwards and burn out this diode.
or perhaps it was installed backwards,
Check out the 1.4 RAMPS wiki to find it on your board.
[reprap.org]
Good news is you can salvage the D1 diode to replace it.
The D1 just allows the Arduino to be powered off the 12V instead of the USB. This is only needed if you want to run off an SD Card
[reprap.org]
Most people wire the 12V backwards and burn out this diode.
or perhaps it was installed backwards,
Check out the 1.4 RAMPS wiki to find it on your board.
[reprap.org]
Good news is you can salvage the D1 diode to replace it.
The D1 just allows the Arduino to be powered off the 12V instead of the USB. This is only needed if you want to run off an SD Card
[reprap.org]