Please peruse this thread:
Nokia Lumia 520 Error: Unable to find bootable opt... - Nokia Support Discussions
Even if your device is not a 520 the information there may apply
First try these steps:
If you are unable to boot the unit and reach the login prompt, then the recovery mode can be triggered to reset the unit by following these steps:
Power off the unit (holding the power button 10 seconds if necessary).
Momentarily press the power button so the unit boots up. You should notice the green LED light next to the front camera. As soon as you see the Nokia logo, hold down the power button until the unit turns off (approximately 10 seconds). You have now successfully interrupted the boot process.
Repeat the above step once more.
Boot the unit for a third time without interrupting the boot process. You will notice the text 'Preparing Automatic Repair' followed by 'Diagnosing your PC'. You have now successfully triggered the recovery mode.
Follow the on-screen guidance.
Secondly there's another reboot sequence described at the bottom of the page I linked you above. It involves pressing the volume rocker and power buttons in a specific sequence. Try that too.
Third, try 'nokia software recovery tool'. It's a simple computer software, download, run and follow steps. you can get it here:
FAQ - FAQ - How can I recover/reset/restore my phone software? - Nokia - UK
If those both fail, my hardware experience would suggest that the most likely thing is the internal memory has stopped working. Any hardware fault like this should be taken in for repairs under warranty. Whatever may happen I am afraid there's a high chance you already lost your data; hope you backed up
Sorry if I couldn't be of help. Just try those things first and let us know how it goes