Please Follow the below mentioned troubleshooting steps to troubleshooting Lync 2010 sign in issues.
- Download and run the auto fix from the Microsoft from here
- Restart "Microsoft online sign in assistance service"
To restart the service, you can follow the below mentioned steps or can restart service from the service.msc or you can do the same by from the Command prompt,
To restart the Service from the command prompt please follow the below mentioned steps.
Open Command Prompt As Administrator.
copy & paste the below command on command prompt.
NET STOP MSOIDSVC & NET START MSOIDSVC
make sure this command run without any showing error message and says successfully restarted "Microsoft online sign in assistance service"
Once the command completed successfully please restart the Machine.
If the issue still not resolved then reinstall the sign in assistance service.
Uninstalling the Sign-in Assistant
In Control Panel, under Programs and features
Select Microsoft Online Services Sign-in Assistant, and then click to Uninstall/Change.
Once the Service is removed then reinstall the same.
Follow the below steps for installing “Sign-in Assistant Service"
Download the Sign in Assistance from here
Once you have the executable downloaded install the same once the installation is finished, restart the machine and then launch the Lync Program.
I hope this will resolve the issue.
*** If there are still any issues make sure to check the user accounts, sip and email address, also the proxy settings etc.