Why doesn't the toilet tank not filling up with water and shutting off?

Because something's wrong with it. Exactly what is hard to tell, and the Wording of the question doesn't help at all.

If it's filling up but not shutting off (it continues to run even after the tank is full, with water going into the overflow drain), the problem is most likely that the shutoff valve needs to be adjusted.

If the water continues to run but the tank isn't filling up (and there's no obvious puddle of water around it indicating a leak), the flapper valve probably isn't seating properly (this is the classic "jiggle the handle" problem).

If the water isn't running at all, it's probably shut off at the wall valve (or somewhere further up the line).