Hello. When im adding this script to player…well its working pretty good,player is flying up,but it will not stop,its even going through ceiling,i mean rigibody which is attached to player does not detect collisions at all,player just go through walls,i need help to fix this,please someone help me 
Physics.gravity = Vector3(0, 50, 0);
Have you added colliders to both the walls and the player ?
50 units of gravity is pretty strong (5 times more powerful than the normal amount), and will make objects move pretty fast. Any small objects will pass through thin colliders (one frame they are on one side of the wall, by the next frame the would have moved far enough to be completely on the other side).
Make the walls thicker, or possibly increase the physics solver iterations