I've got exactly the same Problem with a Ramps1.4 RepRap Prusa i3
Arduino needs 5V on USB to start 
Also just happend "over night".
Was working perfectly with external PowerSupply and then it just did not react at all, LCDisplay dark, ...
I'm currently using StepDownElectronics with USB-Socket connected to Arduino, ... :-/
Any Chance you could Post a Picture of the component you replaced?
Thanks a Lot
Robin