Big-IP 6900 stuck at GRUB stage - no image to choose from

I found this from digging around devcentral, you can this:

Reboot - Esc+shift+9 to get the AOM menu, then select option 4 to bounce the AOM (and the host system as well). The system will go down to a POST and as it is doing the memory test, hit 'b'. This will get you to a boot action menu. Select "PXE boot" from this and make sure you have a PXE server set up. Alternatively you can just boot from a thumbdrive or DVD with a V11.x image on there. Perhaps that will get you going?