Have you thought of hardware problems? try unplugging the sata cable from both the motherboard and harddrive, and unplugging the power on both ends.

then plug it all back in, try booting. You have ruled out a bad connection.

Next try another sata cable, maybe its broken. Try another sata port on the motherboard.

Try and attach it to another machine, maybe through a usb external drive. See if it shows up.