As things are right now, only your player can detect collisions. For this to happen your player must have a collider attached, and be sure the `IsTrigger` checkbox is unchecked. If you want to make it react to `IsTrigger` than you should change your `OnCollisionEnter()` to `OnTriggerEnter()`.

Collisions can only be detected if at least one of the colliding objects has a call to `OnCollisionEnter()` or `OnTriggerEnter()`, depending on if you want the collider to react to collisions or triggers, and obviously both colliding objects must have a collider attached.

For further detail on understanding how to properly handle collision detection among your objects, please see [this question][1].


  [1]: http://gamedev.stackexchange.com/questions/127853/how-to-decide-which-gameobject-should-handle-the-collision