wo is two
you were faster.
So did you check the power and data cables to the hard drives if they are seated properly in both the hard drives' and the motherboard's connectors? If they are, you should check if the drive powers up at all (put your hand on it and feel if it starts resonating). Can you hear strange clicks.
I just realized that when it comes to faulty hard drives, your senses other than vision are the ones which come handy
If nothing is strange about the disk, it still can be the cables' or the motherboard's fault (not likely, but cannot be excluded). I would try it out in another computer, or in an external USB enclosure.
Hmm... actually, the fact that at first they went missing together, doesn't make it likely that the drives are wrong. What are the chances that two drives go wrong at the very same moment...