Unable to log in to the Microsoft Lync desktop client

First, ensure Lync is shut down completely and isn't resident in the system tray.

Occasionally the Lync folder will contain a corrupt Lync profile. To clear this and allow Lync to rebuild the profile, open Windows Explorer and delete the folder:

C:\users\%username%\AppData\Local\Microsoft\Communicator

Restart Microsoft Lync and log in with your email address and password.