Tkinter: button or entry not appearing

Python Help
1

Hello,

On my code when I use a button/entry command using the Tkinter python library my screen appears black.
Here is the first few lines of the code:

import random
from time import sleep
from tkinter import *
import threading


tk = Tk()
HEIGHT = 1050
WIDTH = 1600

canvas = Canvas(tk, width=WIDTH, height=HEIGHT)

tk.title("Times Tables game")
canvas.pack()

class Game:

   def Error():
       raise Exception("There was an error")

   def Start():
       global numOfQuestions
       global questionsClick

       questionsButton = Button(tk, text="How many questions would you like?", 
       command=questionsClick)
       numOfQuestions = Entry(tk)
       questionsButton.pack()
       numOfQuestions.pack()

Thank you.

Ps canvas.mainloop() is at the end