Timer & Isgrounded Issue (Getkey not working)

Goal is to have a "Hold Space to jump higher" that cuts out after the timer runs out & can only be initiated if the player was Grounded to begin with.

2 problems I'm having.

1. When using this Timer script with GetKey (constant Addforce no longer works even if I remove the IsGrounded & the Timer)

2. I can't figure out how to incorporate Is IsGrounded into the timer

3. (I would alse prefer to incorporate a reflect.Addforce with a contact check but I have not got a script even close to working yet)

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class PlayerController : MonoBehaviour {

 public float jumpForce;
 public float TIMER = 0f;
 public float JumpCoolDown = 4f;
 bool IsGrounded;

 void Start ()
 {
     rb = GetComponent<Rigidbody> ();
     IsGrounded = true;
 }

 void FixedUpdate () 
 {
     if (TIMER >= 0f) {
         TIMER -= Time.deltaTime;
     }
 

     if (TIMER < 0f) {
         if (Input.GetKey (KeyCode.Space) && IsGrounded == true) {
             jump ();
             Debug.Log ("Space working");
         }
     }

 }

     void jump ()
     {
         rb.AddForce(Vector3.up * jumpForce);
         TIMER = JumpCoolDown;
     }

 

 void OnCollisionExit ()

 {
     IsGrounded = false;
 }

 void OnCollisionStay (Collision other)
 {

     if (other.gameObject.CompareTag ("Ground")) 
     {
         Isgrounded= true;
     }
 }

}