Itunes Password Not Recognized

I continue to receive a popup windows from iTunes asking me to login. I input my password, the screen goes away, and then pops back up. I know the password is correct because i have used the same password to successfully log in to icloud. I also know its correct because when i use the wrong password, i get an error message from itunes. This window is preventing me from being able to shut my computer down. I tried updating the computer software but that didn't fix the problem.