hi nergal, and welcome to sevenforums,
we feel your pain!
possibly the fall jarred your hdd, and it's not quite connected properly.
do you know how to access the hdd? there should be a panel on the underside of the machine which is easily unscrewable - (look for a symbol which looks like a cylinder).
unscrewing the panel should reveal the hdd - pull the drive out, and push it back in again to re-establish the connection.
screw panel back on, cross your fingers, pray to your favourite deity and reboot.
...or you may need to buy a replacement 2.5" drive.