Tkinter not working in pycharm

from tkinter import *


def main_screen():
    screen = Tk()
    screen.geometry("350x250")
    screen.title("Notes 1.0")
    Label(text="Notes 1.0", bg="grey", width="300", height="2", font=("Calibri", 13)).pack()
    Label(text="").pack()
    Button(text="Login", height="2", width= "30").pack()
    Label(text="").pack()
    Button(text="Register").pack()
    screen.mainloop()
    main_screen()
It is not opening a new window, that is supposed to work buy I don't know why its not. Someone please help.