Enemy collision in 2D top down

We (my friends and I) are trying to make our player take damage when he collides with the enemies, but needless to say we are having some difficulties.
The player stops when colliding with the enemy but the health doesn’t go down. Please help?

Note that we are beginners and haven’t attempted a project of this size before.

using UnityEngine;
using System.Collections;

public class PlayerHealth : MonoBehaviour 
{

	public GUIText healthText;
	public GUIText dieText;
	private int health;
	public float forceSpeed = 0.01f;
	public float speed = 5f;

	void Start ()
	{
		health = 12;
		SetHealthText ();
		dieText.text = "";
	}

	void FixedUpdate () 
	{
		if(Input.GetKey ("s"))
		{
			transform.Translate(-Vector2.up * speed * Time.deltaTime);
		}
		if (Input.GetKey ("w")) 
		{
			transform.Translate(Vector2.up * speed * Time.deltaTime);
		}
		if (Input.GetKey ("a")) 
		{
			transform.Translate(-Vector2.right * speed * Time.deltaTime);
		}
		if (Input.GetKey ("d")) 
		{
			transform.Translate(Vector2.right * speed * Time.deltaTime);
		}
		
		float moveHorizontal = Input.GetAxis("Horizontal"); 
		float moveVertical = Input.GetAxis("Vertical"); 
		
		Vector3 movement = new Vector3(moveHorizontal, moveVertical);
		
		rigidbody2D.AddForce(movement * forceSpeed);
	}

	void OnCollosionEnter(Collider other)
	{
		if(other.gameObject.tag == "Enemy")
		{
			other.gameObject.SetActive(false);
			health = health -1;
			SetHealthText ();
		}
	}

	void SetHealthText ()
	{
		healthText.text = "health" + health.ToString();
		if(health == 0)
		{		
			dieText.text = "YOU DIE!";
		}
		
	}

}

I spot one problem. You are using the 3D version to detect collisions. For 2D, use OnCollisionEnter2D():

void OnCollosionEnter2D(Collision2D coll)

The answer by robertbu is correct. However, there might be more problems at the same time:

I haven’t tried Box2D implementation in Unity yet, but what i can say now is that you are using transform.translate for a rigid body, which in some physics engines, if you translate the body manually, it won’t produce correct collisions. To correctly move a rigidbody you should use forces, and velocity for kinematic bodies (that’s said in Box2D docs, thouugh using velocity in non-kinematic bodies should work too)

Also, you didn’t mention having a rigidbody or a collider (2D versions) on your GameObject, so make sure both are there… OnCollosionEnter2D is a callback of physics engine