What can I do if my Nokia Lumia does not switch on or is unresponsive? - Nokia FAQ
Press and hold the Volume Down key and Power key simultaneously until you will feel vibration. The phone will reset and boot up automatically. Note that this does not reset your Microsoft (Windows Live) Account or remove any of your data.
If your phone still does not respond, remove the SIM card and try again.
When the phone switches on, confirm that it has the latest software version by going to Settings > Phone update, and then tap Check for updates.
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