Lumia 520. Will not power up. message says "unable to find bootable option"

[Windows Central Question]

Lumia 520, had to be switched off and will not power up again Nothing happens when using just battery power but when connected to mains power get just a blank screen and message "unable to find bootable options"

 

[Maurizio Troso]

Lumia 520, had to be switched off and will not power up again Nothing happens when using just battery power but when connected to mains power get just a blank screen and message "unable to find bootable options"

Your phone got a hardware fault. It needs assistance.
You could try flashing original o.s. via some flashing tools like Lumia Phone Test Application, but it works only sometimes.
Try first the hard reset key sequence