Why my character wont jump???

Code:

     var speed : float = 6.0;
     var jumpSpeed : float = 8.0;
     var gravity : float = 20.0;
     var AirSpeed : float = 0.25;
     
     var FireParticle : GameObject;
     
     private var moveDirection : Vector3 = Vector3.zero;
     
     function Update() {
         
         animation.CrossFade("Idle");
     
         var controller : CharacterController = GetComponent(CharacterController);
         
         transform.Rotate(0,Input.GetAxis("Horizontal"),0);
         
         if(Input.GetAxis("Vertical")){
         animation.CrossFade("Run");
         animation["Run"].speed = Input.GetAxis("Vertical")*3;
         }
             moveDirection = Vector3(0,0,Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
         if (controller.isGrounded) {
             if (Input.GetKey(KeyCode.Space) && controller.isGrounded) {
                 moveDirection.y = jumpSpeed;
             }
         }
 
         moveDirection.y -= gravity * Time.deltaTime;
         
 
         controller.Move(moveDirection * Time.deltaTime);
     }