I’m using RAMPS and Arduion Mega too but I never saw this problem before. Maybe your voltage regulator was not able to absorb too much current from RAMPS. You may find the solution here because someone got the same problem as you and they mentioned that endstops caused the short circuit (broken endstop with 0 Ohms) and also pointed that removing the D1 diode from RAMPS could fix the fried regulator problem but you have to power your arduino from USB
(sorry if posting another site url is prohibited here)