This often occurs after you have installed a new machine.  If you failed to reauthorize your old computer, iTunes believes it is still using your music library.  Many times, if you have authorized less than 5 machines, you just let it use another authorization.  But once your 5 authorizations are used up from PCs or Macs, it no longer allows authorizations.

The solution requires you to reauthorize all your machines and reauthorize the ones you still use.

After you authorize more than one computer, you’ll have the option to deauthorize all computers for your Apple ID. You can deauthorize all computers once a year.

You won’t delete anything if you choose to deauthorize all computers. You just won’t be able to play content that you bought from iTunes on those computers.

Here’s when you should deauthorize all your computers:

• If a computer doesn’t work anymore
• If you don’t have one of your computers
• If you used 5 authorizations and want to authorize a new computer

To deauthorize all your computers, follow these steps using one of your authorized computers:

Open iTunes and click iTunes Store

.

Click Sign In and enter your Apple ID

Click your Apple ID and select Account

Click Deauthorize All

After you deauthorize all, you can easily authorize computers again one by one. Just follow the steps to authorize a Mac or PC.