Hello,
Over the weekend, there was a problem with one of our authentication servers. You may have received the following error message:
"Disconnect: This Steam account does not own this game. Please login to the correct Steam account."
This issue has since been corrected.
Please exit from Steam and delete a file in your Steam directory called:
ClientRegistry.blob
This file would be in the location:
C:\Valve\Steam\ClientRegistry.blob or C:\Program Files\Steam\ClientRegistry.blob
(or the directory you installed Steam to).
The next time you start Steam it will re-create this file, do a brief update, and then you should be able to connect and play.
We apologize for any inconvenience this issue has posed.
Please let us know if you have any further questions.