iTunes won't recognize Apple ID
iTunes on my computer won't let me sign in to the iTunes store or let me authorize the computer for my account. When I try to do either it says "The Apple ID you entered couldn't be found or your password is incorrect", but it's correct. I changed the password, updated both my computer software and iTunes itself and it still does it. Any way to fix it?
iMac, OS X Yosemite (10.10.4), iTunes