It may be caused by mechanical or physical problem such as the spindle motor system failure. This failure is usually caused by over-heating. The activity light flashing that you see on your CPU (central processing unit) indicates that its head moves to different tracks. If this movement fails, it can no longer read or write data. Remember, a hard disk is a hardware component, not a software. It is a magnetic device use to store data, it reads and writes data as it spins. Failure of logical system (the logical partitioned drives assigned as C, D, E, F, G) may be caused by and not limited to corrupt system files, corrupt firmware, registry components and virus infection. Spywares, adwares & all other malicious files may also cause your hard disk to crash. (Article continued

If you see the activity light flashing but the computer does not boot, the case may be, your files is still intact on the hard drive but becomes inaccessible -- this may be because of logical system failure. Try booting through a floppy disk or use the start-up disk that came with the system. If you don't see any activity light flashing on the CPU and that the system can not start up then it is likely of mechanical problem.The good news is, usually, in today's technology on hard disks, if this problem occurs it normally will alarm you and warn you before it will totally fails, so you could get that chance to immediately back up your files.