you didn't get signed in. It might be your sign-in adress or logon credentials. so try those again. if that doesn't work, contact your support team

Few days ago I came across an issue in my organization that some lync clients were unable to login. their client prompting for credentials again and again.  I have Lync Server 2010 environment.
If any user Lync client prompting for password and not connecting then first

(1)                Check network connectivity.


If the network connectivity is fine then go to  the following locations, delete the “Communicator” folder and restart the system.


(1)    "C:\Users\user\AppData\Local\Microsoft"

(2)    "C:\Users\user\AppData\Roaming\Microsoft"

(3)    “C:\Users\user\AppData\LocalLow\Temp\Microsoft”
If still the issue not resolved then make sure that the above "communicator" folders or delete and reinstall the lync client on their system.


for further investigation go to cmd and check that from where this client system is authentiacted. go to that DC and check this user lastlogon time, through "net user userid"
also check client and dc time due to time may also this issue occure.