I can’t get the iTunes login to go away. I can’t use or turn off my Mac.

I sat down to my Mac today and the iTunes Login popped up. I tried to exit, but it won’t. I tried to sign in, but it says my password is incorrect. But it works on my iPhone and my iPad. I can’t turn my computer off because iMusic won’t shut off. I have work to do on my computer. Can someone please help me!