According to the company's blog post, the downtime of the service is due to problems with the 'supernodes'. 'Supernodes' are computers which work like phone directories, except that they do so for Skype, directing and determining the connection from one Skype user to another. There are large number of supernodes available under normal situation, but today, it seems that many were offline due to "a problem affecting some versions of Skype".
New ‘mega-supernodes’ are created as we speak to return operations to normal though it may take hours. Some features such as group video calling may take longer.