Pixhawk 2.4.8 Main LED Not Blinking

Hardware Pixhawk 2
6

The LED meanings are a bit hard to find but here they are:

https://discuss.ardupilot.org/t/pixhawk-led-definitions/1139/4

I would try holding down the safety switch while powering up the Pixhawk to force a reload of the I/O firmware:

http://ardupilot.org/copter/docs/common-safety-switch-pixhawk.html#using-the-safety-switch-to-force-i-o-board-firmware-update

Also it seems that the SD card can affect the boot up so try re-seating it or booting without it.