It could be different things. Just play around with the hard drive. In the C840, there's only 1 screw and a latch to remove it. Remove it and if you have an extra 2.5" HDD try putting it in and see if you get past the BIOS post, if not then just take it off and put it back in. But from what I recon, since I had a C840 myself, you have a bad HDD, you can buy a relatively cheap replacement on eBay.