My python program doesn't run the way it should. It runs but doesn't show anything, while it should actually ask for a user input

Question

import string
import random
from words import words

def get_valid_word(words):
    word = random.choice(words)
    while '-' or ' ' in word:
        word = random.choice(words)

    return word.upper()

def hangman():
    word = get_valid_word(words)
    word_letters = set(word)
    alphabet = set(string.ascii_uppercase)
    used_letters = set()
    user_letter = input("Guess a letter: ").upper()
    if user_letter in alphabet - used_letters:
        used_letters.add(user_letter)
        if user_letter in word_letters:
            word_letters.remove(user_letter)
    elif user_letter in used_letters:
        print('You have already used that character. Please try again')

    else:
        print('Invalid character. Please try again')
 hangman()

#This is the code for my program. When I run it should ask for a user input "Guess a letter", but it doesn't do so and just runs forever.