A harddisk is made of magnetic disks, just like a video tape, which are read using an arm, just like a record player. It also contains some electronics, that handle error correction and makes it possible to connect a standard interface to the harddisk.
The most common failure is either mechanical, i.e. the reader arm may fall off or just touch the sensitive magnetic surface or that parts of the magnetic surface becomes unreadable. The internal electronic may also fail.
All of these errors makes it impossible to access the data via the standard interface. In reality, most of the data still exists. It just cannot be read easily. There are companies which have specialized in saving data from such situations. They take the disk apart inside a special clean room and uses special machines to read the disks. This is very expensive and out of the range of ordinary people.