my lumia 520 says unable to find a bootable option when i press that start button

[WPCentral Question]

i was using whatsapp on my lumia 520 when suddenly the phone switched off and now when i am trying to switch on the phone it says "ERROR: Unable to find a bootable option. Press any key to shut down". i tried using the volume down key n the power button but it isn't working and also that my volume up button isn't working. PLEASE HELP.
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[Tung Nguyen5]

I was like you so, can not update my 520 up window 8.1