Google is having issues with it's Google Talk service. For about three hours now, Google Talk has been down. At this point, Google is investigating the matter and has promised an update about 10 minutes from now. Google hasn't provided a time frame for the service to return and they haven't provided a reason for the outage.
The status of Google Talk can be checked on Google's website. We will continue to keep you updated as we learn more about the outage, its causes, and when the service comes back. Until then, you're going to have to find another way to chat with your friends.