It is down indeed (http://heartbeat.skype.com/2015/09/skype_presence_issues.htm... and Twitter https://twitter.com/Skype/status/645919917510914048). Desktop client might be working for some people, and I don't know if the mobile apps are affected too.

The desktop client isn't working for me, but http://web.skype.com seems to work.

Same here, can't log in.