In order to fix the issue, press Alt + Ctrl + Delete key combination, open Task Manager and close all Skype instances. If you can't, reboot the computer, press the F8 key to enter in Safe Mode and reset Skype settings by renaming the %appdata%\Skype
folder (you can access it using the search engine from the Start menu) and deleting the DbTemp directory. Check the official forum for further information.