SpringJoint2D Cannot be referenced properly

So I was trying to make my joint, and when the player stops holding with his mouse, a projectile would be fired from the sling.
and i get this problem when releasing the projectile from the sling :

NullReferenceException: Object reference not set to an instance of an object
SlingBackController.ShootProjectile ()

as if i haven’t referenced anything for my breakforce to do but i did from this script

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

public class SlingBackController : MonoBehaviour
{
    public float MaxDistance = 5f;
    public Transform MiddlePosition; 
    private static SlingBackController _instance;
    private SpringJoint2D CurrentSpringJoint;

    public static SlingBackController Instance
    {
        get
        {
            if (_instance == null)
            {
                var obj = GameObject.FindGameObjectWithTag("SlingBack");
                if (obj != null)
                {
                    _instance = obj.GetComponent<SlingBackController>();
                }
            }
            return _instance;
        }
        set
        {
            _instance = value;
        }
    }


    // Start is called before the first frame update
    void Awake()
    {
        Instance = this;  
    }

    void start()
    {
        CurrentSpringJoint = GetComponent<SpringJoint2D>();
    }

    // Update is called once per frame
    void Update()
    {
        
    }

    public void ShootProjectile()
    {
        CurrentSpringJoint.breakForce = 1f;
        CurrentSpringJoint = null;
    }
}

the spring was clearly referenced and shown as a SpringJoint2D, and it still doesn’t break when player releases the projectile through my script, the problem is pointing to the ShootProjectile() problem, Can anyone please help?

Start() function does not get called because you forgot to capitalize the “s”.