Due what they are calling a Peer to Peer Network outage Skype users are finding it impossible to log in.
Suggestions to those who need to be in touch are to use the following programs as they are dual platform (Windows and Mac):
1) SightSpeed http://www.sightspeed.com/
2) GizmoProject http://www.gizmoproject.com/
What seems to be the issue is login authentication, not the Skype network. A check with some colleagues and friends who didn't log off of Skype shows that if you're on the network, you'll still be logged in. Since this happened over night many here in the USA seem to be the ones who will be most affected.
The folks who get hurt by this are the Skype based service providers who need the Skype connectivity layer to keep things working.
Hopefully Skype gets things up and running by the middle of the work day on the east coast.
You can check the Skype status at their Heartbeat page here.
What are you using today instead?
Update-Tom Keating speculates that there may be some ties to a Windows update. Wouldn't that be too funny, if Skype was disabled by a patch made by Microsoft. Ouch.