If you are connected to the internet, Skype services are running as expected, the software is installed properly and you are using the right credentials, then there are only a few things that could still be stopping you from signing in:

☛ Delete the shared.xml file.

☛ Check that the ports which are required by Skype are open.

☛ Check if Skype is already running in the background.

☛ If you connect through a proxy, make sure that Skype can connect to your proxy server.