What does "Restricted" (aka. "Do not") system icon on Lollipop mean? I'm using Nexus tablet on stock Lollipop. For the life of me, I can't find what the "restricted" sign in the top right corner means. I've checked the user manual as well but still not worked out what it means.

What is the meaning of that icon?
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").