Having trouble restarting a countdown timer using time.deltatime

Hi, please help. I've been having trouble with this problem for far too long.
I'm having trouble restarting a countdown timer back to 30 seconds. It works the first time it counts down to 0 but afterward, when I try to restart it, it doesn't work.

Here is my code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.SceneManagement;

public class CountdownTimer : MonoBehaviour
{
float currentTime;
float startingTime = 30f;

Vector2 restartPosition = new Vector2(0f, -9.25f);
public GameObject player;
public PlayerMovement PlayerMovement;
[SerializeField]
Text countdownText;
public AudioSource froggerDied;

public bool restartTime;

// Start is called before the first frame update
void Start()
{
currentTime = startingTime;
}

// Update is called once per frame
void Update()
{
countdownText.text = currentTime.ToString("0");
currentTime -= Time.deltaTime ;

Debug.Log("currentTime = " + currentTime);

if (currentTime > 5)
{
countdownText.GetComponent<Text>().color = Color.green;
}
if (currentTime < 5)
{
countdownText.GetComponent<Text>().color = Color.red;
}
if (currentTime <= 0)
{
player.transform.position = restartPosition;
PlayerMovement.froggerLives -= 1;
currentTime = startingTime;
froggerDied.Play();
}
if (restartTime == true) //restartTime is called true in another script but this shouldn't be a problem
{
currentTime = startingTime;
restartTime = false;
}
}
}