What does "Restricted" (aka. "Do not") system icon on Lollipop mean?
It means that you (might be accidentally) set the "Interruption Mode" to "None". It means that no calls &amp; notifications will be shown, and not even alarm will ring.

This is part of Interruption Mode, a new feature in Android 5.0 Lollipop. You can access this setting from Settings - Sound &amp; notification - Interruptions - When calls and notifications arrive.

Alternatively, you can use volume keys as a shortcut to access this feature ("None", "Priority", or "Always").