Bmx tire hole. Won't pump up!
Take the tube out and find out why its leaking. A picture or description of the tube damage may help diagnosis. 

A variety of things could have caused this:


Something else stuck in the tire which damaged the tube
Pinching the tube between the rim and the tire
Damage at the valve
A bad tube
A spoke coming through and punching the tube
Damaged Rim Tape
Etc.


If there is an appreciable hole or damage in the tire, it won't protect the tube and you need a new tire.

Recommended reading: Sheldon Brown