The crankshaft pulley is the pulley driven by the crankshaft. It runs a belt that runs around the pulleys of all of you accessories. You have pulleys for your alternator, power steering pump, a/c compressor, and any other accessories.
If your crankshaft pulley becomes disengaged, you lose your ability to drive all of the other accessories, so you will lose power steering, your alternator wont charge your battery, your a/c wont work, and eventually you will be left on the road stranded.
I hope this helps, let me know if you have any more questions!
If your belt drives the water pump, then yes it can cause it to overheat.
And yes, if your belt comes off your alternator pulley, you wont charge your battery so you will eventually run down your battery and it will stop running.
I hope this helps!