I don't want to worry you but the same thing happenend to one of my laptops, until it finally failed. Carefully reattach the HD as Duke said and run some deep scan on your hd.
Rig
*Corei5 3570K *Nvidia 9800GT 1GB DDR3
*Motherboard Asus P8H77M-PRO *Win7 x64
*RAM 8GB DDR3(1600)
