I am sorry if I am misunderstood. I did not say your teminology is incorrect. That was just my assumption of 3 g.
Anyway, Fn key should not be necessary for the function of F8 key. But you say F8 key is not working, either.
In this case, you do not have much choice left but (1) taking the unit to a repair shop for keyboard replacement ( check if you still have warranty ) (2) trying another keyboard. If this is a notebook, then you can connect an USB ( wired or wireless ) external keyboard and hope it works. If this a desktop PC, then you have to use an PS/2 keyboard because your legacy USB support might have been disabled in BIOS so you cannot use Fn key or F8 key before Windows is not loaded.