Hard Drive should be easily accessible from the center panel on the bottom of the laptop. A single screw holds the center panel in place. Hard drive should be visible near the front of the laptop once panel is removed.
If you have been testing gravity with your laptop, it may have just been knocked loose. With the laptop completely unplugged (remove battery & power cord), start by unplugging the HDD, and plugging it back in. Now cross your fingers and turn it back on.
If that doesn't do it, you'll want to go into the BIOS settings and check to see if the hard drive shows up anywhere under hardware, or boot options.