CTRL and Right Click on the Lync icon in your task bar and select Sign-in Logs
If the window states “no sign-in data available” that’s simply because the sign-in process was successful.
To collect the logs click on the Copy to Clipboard button and paste that into a text editor of your preference
In my example here you will see 2 reported errors.
The first states a trust issue and the second mentions that the client isn’t authorized.
The underlying cause, invalid credentials.