Is it a desktop PC or a laptop? When starting up the computer as soon as you press the power button, start tapping the escape key until the boot menu is brought up. Click on BIOS or System Settings (should be F10). In there you can look on first tab page to see if your hard drive is showing up. If not, go over to bootable devices within the BIOS and see if the drive is recognized.
Also, make sure that the SATA and power cables are securely plugged into your hard drive and the motherboard (if it's a desktop). Report back with your findings as usually, if it's not a loose cable, and the drive is still showing up in the BIOS, then the drive itself might not be damaged; your copy of Windows might have just become corrupted and therefore will need to be repaired or restored.