I have been trying to get a working isGrounded variable to get my character to stop from infinitely jumping can someone please show and/or explain what it is that I am doing wrong.

public class Player : MonoBehaviour {

 [SerializeField]
 private float speed = 10.0f;
 private float horizontalInput;
 [SerializeField]
 private float verticalInput;
 [SerializeField]
 private float jumpHeight = 5.0f;
 private Rigidbody2D myrigidbody2D;
 private bool isgrounded;
 float Ypos;
 float YposVerify;

 // Use this for initialization
 void Start()
 {
     myrigidbody2D = GetComponent<Rigidbody2D>();
     isgrounded = false;
 }

 // Update is called once per frame
 void Update()
 {
     Movement();


     if (Input.GetKeyDown(KeyCode.Z) && isgrounded == true)
     {
         //transform.position += new Vector3(transform.position.x, transform.position.y + 20.0f,0);
         myrigidbody2D.velocity = new Vector2(0, speed * jumpHeight);

     }

 }


 private void Movement()
 {
     transform.Translate(Vector3.right * Time.deltaTime * speed * horizontalInput);
     horizontalInput = Input.GetAxis("Horizontal");


 }

 //make sure u replace "floor" with your gameobject name.on which player is standing
 void OnCollisionEnter(Collision theCollision)
 {
     if (theCollision.gameObject.name == "Ground")
     {
         isgrounded = true;
     }
 }

 //consider when character is jumping .. it will exit collision.
 void OnCollisionExit(Collision theCollision)
 {
     if (theCollision.gameObject.name == "Ground")
     {
         isgrounded = false;
     }
 }

}