Unity: Pausing the game after player object gets destroyed

Hey guys, as the title says, I want to make it so that when the player game object gets destroyed, the game will then pause and bring up a screen (I've made one called GameOverScreen), however, I cannot seem to get my head around it. The screen displays however the game doesn't pause. Any idea what could be causing this? Here is my code:

using UnityEngine;
using UnityEngine.UI;

public class PlayerHealth : MonoBehaviour
{
public float m_StartingHealth = 100f; // The amount of health each tank starts with.

public Slider m_Slider; // The slider to represent how much health the tank currently has.

public Image m_FillImage; // The image component of the slider.

public Color m_FullHealthColor = Color.green; // The color the health bar will be when on full health.

public Color m_ZeroHealthColor = Color.red; // The color the health bar will be when on no health.

public GameObject m_ExplosionPrefab; // A prefab that will be instantiated in Awake, then used whenever the tank dies.

public Camera mainCamera;

public Camera gameOverCamera;

public GameObject GameOverMenu;

private bool gameover = false;
private AudioSource m_ExplosionAudio; // The audio source to play when the tank explodes.
private ParticleSystem m_ExplosionParticles; // The particle system the will play when the tank is destroyed.
private float m_CurrentHealth; // How much health the tank currently has.
private bool m_Dead; // Has the tank been reduced beyond zero health yet?

void Start()
{
GameOverMenu.SetActive(false);
}

private void Awake()
{
// Instantiate the explosion prefab and get a reference to the particle system on it.
m_ExplosionParticles = Instantiate(m_ExplosionPrefab).GetComponent<ParticleSystem>();

// Get a reference to the audio source on the instantiated prefab.
m_ExplosionAudio = m_ExplosionParticles.GetComponent<AudioSource>();

// Disable the prefab so it can be activated when it's required.
m_ExplosionParticles.gameObject.SetActive(false);
}


private void OnEnable()
{
// When the tank is enabled, reset the tank's health and whether or not it's dead.
m_CurrentHealth = m_StartingHealth;
m_Dead = false;

// Update the health slider's value and color.
SetHealthUI();
}


public void TakeDamage(float amount)
{
// Reduce current health by the amount of damage done.
m_CurrentHealth -= amount;

// Change the UI elements appropriately.
SetHealthUI();

// If the current health is at or below zero and it has not yet been registered, call OnDeath.
if (m_CurrentHealth <= 0f && !m_Dead)
{
{
gameover = !gameover;
}


if (gameover)
{
GameOverMenu.SetActive(true);
Time.timeScale = 0;
}

if (!gameover)
{
GameOverMenu.SetActive(false);
Time.timeScale = 1;
}
OnDeath();
mainCamera.transform.parent = null;
mainCamera.enabled = true;
gameOverCamera.enabled = false;

}
}


private void SetHealthUI()
{
// Set the slider's value appropriately.
m_Slider.value = m_CurrentHealth;

// Interpolate the color of the bar between the choosen colours based on the current percentage of the starting health.
m_FillImage.color = Color.Lerp(m_ZeroHealthColor, m_FullHealthColor, m_CurrentHealth / m_StartingHealth);
}


private void OnDeath()
{
// Set the flag so that this function is only called once.
m_Dead = true;

// Move the instantiated explosion prefab to the tank's position and turn it on.
m_ExplosionParticles.transform.position = transform.position;
m_ExplosionParticles.gameObject.SetActive(true);

// Play the particle system of the tank exploding.
m_ExplosionParticles.Play();

// Play the tank explosion sound effect.
m_ExplosionAudio.Play();

// Turn the tank off.
gameObject.SetActive(false);
}
}