Hmmm... You don't give too much information to start with.
You have played successfully with the computer in question, haven't you? If so, you might want to try one or several of these tricks, in this order:
- Unplug your modem from mains and replug to reset your Internet connection
- Restart your computer, if you have used Hibernation and Sleep mode for a long time
- Do System Restore to a date you know you could run the game
- Save your Settings folder and reinstall the game