Why isn't my character jumping??

Hi Guys, I am relatively new to Unity and am wondering why I cannot get my player to jump. This is the code I have so far, any suggestions help :)

 CharacterController characterController;

 Rigidbody rb;

 public float jumpVelocity;

 private void Awake()
 {
     characterController = GetComponent<CharacterController>();
     rb = GetComponent<Rigidbody>();
 }

 private void Update()
 {
     if (characterController.isGrounded && Input.GetButtonDown("Jump"))
     {
         Debug.Log("Jump");
         rb.velocity = Vector3.up * jumpVelocity;
     }
 }