Re: SRX - No bootable media found

This looks like your boot partition was corrupted in a power failure.  You can use these instructions to reinstall Junos from this loader prompt.

http://www.juniper.net/techpubs/en_US/junos14.1/topics/task/installation/security-junos-os-boot-loader-usb-storage-device-srx-series-device-installing.html

Once running again, you should take snapshots to create a backup boot partition on the SRX.  Then when this happens the SRX can still boot to the backup partition.

request system snapshot slice alternate