After shutdown for a few days - black screen when booting

I shutdown my freenas for a few days and when I started it again I was not able to connect to any of my webinterfaces. Also ssh'ing into it was not possible.
So I plugged in a monitor and keyboard and I saw a blackscreen with a blinking cursor.
When I restart the freenas after the bios when the boot process starts the blackscreen with the blinking cursor appears again.

I have my boot usb drive mirrored but even when the drive is dead, should't there be something like "can't read boot...

After shutdown for a few days - black screen when booting