PlayerJump Code

i try this code out i got no error’s but my player won’t jump what im i doing wrong

using UnityEngine;
using System.Collections;

public class PlayerJump : MonoBehaviour {

public float jumpSpeed = 20;
public float gravity = 9.8f;
CharacterController controller;
Vector3 currentMovement;

// Use this for initialization
void Start () 
{
	controller = GetComponent<CharacterController>();
}

// Update is called once per frame
void Update () 
{
	if (!controller.isGrounded)
		currentMovement -= new Vector3 (0, gravity * Time.deltaTime, 0);
	else 
		currentMovement.y = 0;

	if (controller.isGrounded && Input.GetButtonDown("Jump"))
		currentMovement.y = jumpSpeed;
}

}

1 Try to add the following:

if (controller.isGrounded && Input.GetButtonDown("Jump")) 
{
    Debug.Log("When debugging code, make liberable use of Debug.Log to find out whether conditions are being met");
    currentMovement.y = jumpSpeed;
}

2 What value have you assigned to jumpSpeed?