iTunes Won't Quit

My iTunes app will not quit and continues to ask for me to sign in. I am unable to restart, logout, update, or shut down my laptop because of this. I continue to sign in to the iTunes tab that pops up saying I must sign in, but it just pops right back up even though I am using the correct password. Even when I press the "not now" button, it pops back up. This is prohibiting me from restarting, updating, and clearing out storage being taken up by iTunes movies. If anyone knows what is going on I would be very thankful.