Unable to restart level in Python Pygame

So, I am making Flappy Bird and when the player dies I have the spacebar assigned to reset the game. However, it's not working and I truly have no idea why (after like 3 hours of going through tutorials and rereading the code).

import pygame
import random

# Initiating pygame
pygame.init()

# Variables:
FPS = 60

WHITE = (255, 255, 255)

ORIGIN = (0, 0)
ground_X_pos = 0

run = True
not_collided = True

BIRD_STARTING_POS = (50, 280)
bird_pos = 0
gravity = 0.3

pipe_list = []
available_pipe_height = [300, 310, 320, 330, 340, 350,
                         360, 370, 380, 390, 400, 410, 420, 430, 440, 450]
SPAWN_PIPE = pygame.USEREVENT

GAP = 200

# Setting the display window
pygame.display.set_caption("Crappy Bird")

SCREEN_DIMENSIONS = (352, 600)
SCREEN = pygame.display.set_mode(SCREEN_DIMENSIONS)

# Loading required images as surfaces
bg = pygame.image.load("assets/sprites/background-day.png").convert()
bg = pygame.transform.scale(bg, SCREEN_DIMENSIONS)

ground = pygame.image.load("assets/sprites/base.png").convert()
ground = pygame.transform.scale(ground, (352, 75))

bird = pygame.image.load("assets/sprites/yellowbird-midflap.png").convert()
bird = pygame.transform.scale(bird, (50, 34))

pipe = pygame.image.load("assets/sprites/pipe-red.png").convert()

_game_over = pygame.image.load("assets/sprites/gameover.png")
_game_over = pygame.transform.scale(_game_over, (352, 77))

# Loading required rectangles
bird_rect = bird.get_rect(center=BIRD_STARTING_POS)

# Spawning pipes
pygame.time.set_timer(SPAWN_PIPE, 1010)


# This will load all the required surfaces of entities
def update(_pipe_list):
    # Loading in background surfaces
    SCREEN.blit(bg, ORIGIN)

    # Loading in the bird
    SCREEN.blit(bird, bird_rect)

    # Loading in the pipes
    for _pipe in _pipe_list:
        if _pipe.bottom >= 600:
            SCREEN.blit(pipe, _pipe)
        else:
            _pipe_ = pygame.transform.flip(pipe, False, True)
            SCREEN.blit(_pipe_, _pipe)

    # Loading in the ground
    SCREEN.blit(ground, (ground_X_pos, 525))
    SCREEN.blit(ground, (ground_X_pos + 352, 525))


# This will spawn in pipes
def spawn_pipe():
    seed = random.choice(available_pipe_height)

    bottom_pipe = pipe.get_rect(midtop=(360, seed))
    top_pipe = pipe.get_rect(midbottom=(360, seed - GAP))

    return bottom_pipe, top_pipe


# Puts the pipe in their correct positions
def pipe_pos(_pipe_list):
    for _pipe in _pipe_list:
        _pipe.centerx -= 5

    return _pipe_list


# This checks if the bird has collided with anything or not
def collision_checker(_pipe_list):
    for _pipe in _pipe_list:
        if bird_rect.colliderect(_pipe):
            return False
        else:
            pass

        if bird_rect.bottom >= 525 or bird_rect.top <= 0:
            return False
        else:
            pass

    return True


def game_over():
    SCREEN.fill(WHITE)
    SCREEN.blit(_game_over, (0, 261.5))


# This is the main loop for running the game
while run:
    # Setting a fixed frame rate
    pygame.time.Clock().tick(FPS)

    # Checking if the game needs to be closed
    for event in pygame.event.get():  # pygame.event.get() gets all events occurring (like key press, mouse clicks, etc)
        if event.type == SPAWN_PIPE:
            pipe_list.extend(spawn_pipe())
        else:
            pass

        if event.type == pygame.QUIT:
            run = False
        else:
            pass

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE and not_collided:
                bird_pos = 0
                bird_pos -= 8.25
            else:
                pass

            if event.type == pygame.K_SPACE:
                bird_pos = 0
                bird_rect.center = BIRD_STARTING_POS
                print("Debug : This works.")
                pipe_list.clear()

                not_collided = True
            else:
                pass
        else:
            pass

    # Running the game
    if not_collided:
        update(pipe_list)
        ground_X_pos -= 5

        if ground_X_pos <= -352:
            ground_X_pos = 0
        else:
            pass

        bird_pos += gravity
        bird_rect.centery += bird_pos

        pipe_list = pipe_pos(pipe_list)

        not_collided = collision_checker(pipe_list)

    #Ending the game
    if not not_collided:
        game_over()

    pygame.display.update()

pygame.quit()

It gets stuck on this screen.