Skype's communications networks relies on "millions of individual connections between computers and phones to keep things up and running", but when a Skype application has trouble connecting to another computer or phone it will turn to a supernode to find the addressing details of the desired connection.
"Unfortunately, today many of [the supernodes] were taken offline by a problem affecting some versions of Skype. As Skype relies on being able to maintain contact with supernodes, it may appear offline for some of you," Skype wrote in the blog post.
Skype is working to fix the problem by creating what it terms "mega-supernodes" to help bring the communications network back online, Skype wrote.
No mention on which versions are effected or whether it is country specific but my version 18.104.22.168 here in New Zealand is down.