There are an enormous number of things which can go wrong with a car - and with so little information no-one can even make an intelligent guess. It really needs a mechanic to examine a vehicle to work out what is wrong with it.

Your stereo could have been the noise, turning the ignition off could be why it shut off.

How many fingers am I holding up? Without any more details how the hell would anyone know?

Could be 101 things. I put my ear up next to my lap-top, but could not hear a thing. I tried to narrow it down to 100, to no avail. Ask a "Pro".

Probably something very expensive. Noise is not cheap.

Almost anything.