With the window already down, when you hit the "down" button, does the motor hum? And, is there no sound at all when you hit the up button? If both statements are true, you probably don't need a new motor, but have a circuit or switch problem. Up and down just reverse the direction of current flow through the same motor. If it runs in one direction, it should run in the other.
If you hear a hum in both directions, youy probably have a mechanism that binds one way.
