Collision2D is not a valid Unity “Magic” method so it is never called unless you handle it yourself. The one you are looking for is most likely OnCollisionEnter2D(Collision2D).
using UnityEngine;
using UnityEngine.SceneManagement;
public class BallScript : MonoBehaviour
{
public Vector2 startForce;
public Rigidbody2D rb;
void Start()
{
rb.AddForce(startForce, ForceMode2D.Impulse);
}
// OnCollisionEnter2D is called when this collider2D/rigidbody2D has begun touching another rigidbody2D/collider2D (2D physics only)
void OnCollisionEnter2D(Collision2D other)
{
if (other.gameObject.name == "wall_bottom")
SceneManager.LoadScene("mainlevel");
}
}
Note that the SceneManager is the new way to manage scenes.