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?
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It means that you (might be accidentally) set the "Interruption Mode" to "None". It means that no calls & 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 & notification - Interruptions - When calls and notifications arrive.
Alternatively, you can use volume keys as a shortcut to access this feature ("None", "Priority", or "Always").
Well, that symbol signifies that you are not getting mobile network from the operator or sometime it also light up when you have switched the blocking mode on, but in your case its defiantly the mobile network, I think you should try removing or reinserting the SIM card also make sure that the SIM card is activated.