I just bought a Samsung Galaxy S yesterday, then all of a sudden today it crashed. The screen is completely black, can't turn it back on, and it has no reaction to being charged... wtf

Can you replace or return it? It sounds defective....

Most Android phones have a battery compartment that can be opened, if with a little difficulty and finesse. Pry that open, remove the battery, put it back in, and turn the phone on, and it'll be like nothing happened.