Hard drives have several mechanical components that can wear down over time. The read/write heads, spindle and actuator assembly are particularly sensitive, and a slight offset will prevent the drive from reading data correctly. Likewise, the hard drive's electronics can fail due to an electrical event or improper operating conditions. This prevents the drive from interfacing with your computer.
User error, virus attacks and file corruption can also cause data loss. When any of these situations occur, you need help from a professional data recovery company in order to restore access to the files on your disk.