Hi i have a problem with the game engine.Please help me.

So i have this problem,i deleted a face of a cube.

But when i press p the cube in game mode disappear.Why?
To solve this problem i only need to press e on edit mode but this is a waste of polygons.
Can you help me solve this problem?

The faces have normals, and they’re pointing outwards, toward the camera. They show up that way. If you don’t invert the normals, then they won’t point toward the camera, and so won’t show up. Alternatively, go into the material and disable backface culling.

To Invert normals: Ctrl-N