Do you have a multimeter? With power off, probe the 12V power input at RAMPS and measure resistance. I read about 100 ohms on mine.

I don't think it takes down the 12V supply, but improperly connected endstops can short the Arduino 5V to ground, shutting down at least the Arduino. We had PM'd about jumpering across the polyfuses on RAMPS - did you do that? If so, maybe that makes a difference on whether 5V problems on RAMPS can affect 12V. Normally the polyfuse would just open in case of an overcurrent.