First you should check to see how many authorizations are currently being used by following the steps in How many computers are authorized to play music from my account?
If the number there is higher than the number of computers that you have authorized, it is possible that one or more of your computers is using multiple authorizations.
To fix this:
- Open iTunes.
- From the Store menu, choose Deauthorize Computer.
- Enter your account name and password.
- Click OK. A message appears reading "This computer has been successfully deauthorized."
- Click OK.
- Repeat steps 2 through 5 above. If you see the same message reading: "This computer has been successfully deauthorized," then your computer was using multiple authorizations.
Repeat steps 2 through 5 above until you see a message reading "This computer was not authorized. To authorize this computer, play a song or video you have purchased using this account."
If you have multiple computers, you can repeat this process on your other computers until the number of Computer Authorizations on your Apple Account Information page accurately reflects the number of computers you have authorized to play music from your iTunes Store account.
To avoid running into this in the future, make sure to deauthorize your computer before you make any major change to it or have it serviced. Also, make sure you remember to deauthorize your computer before you sell it or give it away.
Note: If you no longer have access to a computer that is using one of your authorizations, you can deauthorize all of the computers associated with your account by following the steps at the bottom of About iTunes Store authorization and deauthorization.