Earlier this week I had a Lync 2010 customer who couldn’t log in. He wasn’t getting any error messages; Lync just continued trying to connect to the server without erroring or timing out. This fixed it:

1. Deleted these two folders:
   %localappdata%\Microsoft\Communicator
   %appdata%\Microsoft\Communicator
2. Open Credential Manager and deleted any credentials that referred to OC or SIP in connection with the Lync domain name.

I could have enabled Lync logging to diagnose exactly what was going on before taking out the hammer, but sometimes you just have to give it a good whack and move on.

The Dogbert school of tech support: shut up and reboot.