def playAgain():
    while True:
        try:
            replay = input("Do you want to play again? ").lower() #Asking user to play again
        except ValueError:
            print("Sorry, Invalid Entry") #If response invalid, will ask again
            continue
        if replay in ("yes","y","true","t"):
            main()
        elif replay in ("no","n","false","f"):
            print ("Goodbye")
            return
        else: #If input is invalid will ask again
            print("Invalid entry, Please enter yes or no")

def main():
    print ("Hello")
    playAgain()

main()

main -> playAgain -> main -> playAgain
                                 ^^^

if replay in ("yes","y","true","t"):
        main()
        return   # here