Hi
Could you confirm that the services are all up and running and the back end database connection are all good. Did you made sure that the server is updated to the latest CU also update the client to the latest version. From the client machine
delete the old profile create a new one and try signing in to the client
Delete local cache files on client computer.
%userprofile%\AppData\Local\Microsoft\Office\15.0\Lync\sip_UserName@Domain.com
Registry Key: HKCU\Software\Microsoft\Office\15.0\Lync\UserName@Domain.com
Linus