Isn't that the OS on the Mac? You may get additional responses if you will allow me to change your thread title and move your thread to a better section of the forum.
Try opening iTunes, going to the the Store menu and authorizing the computer. That's my first guess. Also make sure you are still logged into your iTunes account, upper right corner.
I'll think about it some more,
But if you are in a hurry I'd just call Apple support. There has to be something simple that went wrong.
Thanks for the help but it was me who caused the problem. Upgraded to Snow Leopard but had not at that time uploaded all the updates. All working fine now. Thanks.