iTunes recurring sign in pop up won't go away

Hi my MacBook has been working normally otherwise and I can sign in to iTunes elsewhere but the iTunes pop up sign in window simply won't go away even though the password is definitely right. Don't dare shut down in case I get what someone else in an old post from few years back describes - namely that they couldn't then re-start their computer because the pop up wouldn't let them move past that stage. At least now it isn't interfering in any other way but it's annoying, and clearly not right and will presumably causes a problem at some point... Previous advice involved restarting in safe mode and putting in lots of details I don't know if I have for this computer....any advice gratefully received.