Player can only jump once!?

Hi, so I'm following a 'gameswithjames' tutorial on a 2d platformer. My player could jump fine right up until two minutes ago where I started to add checkpoints and respawning in part 3 of the tutorial.

Now I just get the one jump off and am grounded for ever afterwards. Again, it worked before without changing any of the player movement script. I can't figure out whats changed.

Any help would be appreciated!

 using UnityEngine;
 using System.Collections;

 public class playerController : MonoBehaviour {

 //  basic movement
 public float moveSpeed;
 public float jumpHeight;

 public Transform groundCheck;   //  transform has a location
 public float groundCheckRadius; //  Determines size of circe
 public LayerMask whatIsGround;

 private bool grounded;
 private bool doubleJumped;

 // Use this for initialization
 void Start () {
 
 }

 void FixedUpdate()  //  fixedupdate occurs a set amount of time every second
 {
     grounded = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);
 }
 
 // Update is called once per frame
 void Update () {

     if (grounded)
     {
         doubleJumped = false;
     }

     //  Jump
     if(Input.GetKeyDown (KeyCode.Space) && grounded)    //  checks to see if space button is 'keyed' down (pressed) and checks if grounded
     {
         Jump();
     }

     //  Jump
     if (Input.GetKeyDown(KeyCode.Space) && !doubleJumped && !grounded)    //  checks to see if space button is 'keyed' down (pressed) and checks if grounded
     {
         Jump();
         doubleJumped = true;
     }

     //  Move right
     if (Input.GetKey(KeyCode.D))
     {
         GetComponent<Rigidbody2D>().velocity = new Vector2(moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
     }

     //  Move left
     if (Input.GetKey(KeyCode.A))
     {
         GetComponent<Rigidbody2D>().velocity = new Vector2(-moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
     }
 }

 public void Jump()
 {
     GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x,        jumpHeight);
 }
 }