Had this a long time ago when I used to download tunes from Tesco music onlne. The license is issued to your PC. If you change your PC after downloading music from an online source it won't recognize the PC the license was issued to, and therefore won't allow you to play it. You will either have to download them again or get in touch with Itunes help desk or whoever you downloaded your music from to see if they are willing to allow you to download again . Perhaps you can just re download them for free, as they usually allow you to do this 3 times after purchase I believe.
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