You might have just fried the D9 MOSFET. The MOSFET is the upright transistor that has a metal tab to which you can attach a heat sink. There are 3 on a RAMPS, one for D8, D9, and D10.If you're handy with a soldering iron, it's not that difficult to replace, and the component itself is relatively inexpensive. If there's nothing else obviously wrong, this is the approach I would take.