There are many reasons that can disturb the Skype functionality. The software can be blocked by security programs or by Windows firewall. Also, it may not be installed correctly, or some of its files may be corrupted or missing. Also, the problem can be caused by your Internet connection. In order to fix the issue, I recommend you uninstall the software and install it again. After that add Skype in the Exceptions list of your antivirus and Windows firewall. If you still have problems, update the software to the latest version.