GetComponent(); Explain?

Question

Unity

In general, now for some reason in any C # script in Unity it does not work. GetComponent<GameObject>();
The script hangs on the Cube, the object is on the game stage, as this function stopped working, the script did. But the value of the variable is still null.

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

public class TestGetComponent : MonoBehaviour {
    private GameObject HeroObject;
    private void Start()
    {
        HeroObject = GetComponent<GameObject>();
    }
    private void Update()
    {
        Debug.Log(HeroObject);
    }
}

PS Correction, it does not work exactly HeroObject = GetComponent<GameObject>();
If you write Then everything is ok. Or if for example:HeroObject = this.GameObject;

private Animator ch_anim;
private void Start()
{
ch_anim = GetComponetn<Animator>();
}

that's ok too.