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When you perform the software update do you have sufficient battery charge? Have you tried to charge the phone for 1 to 2 hours? You can try to press and hold the power button of the phone for 10 seconds until you feel vibrations an then try to switch on your phone.
If still won't work, best to send your phone to repair for assessment. You can click on Nokia Support on the bottom of this page for further assistance how to send the phone for repair.
Hope this will help you.
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