An electrical problem such as a printed circuit failure or mechanical failure such as a head crash or servo problem.
A software problem such as bad partition tables, file allocation tables or crashed Netware volumes.
A virus has infected your computer or someone has sabotaged your system.
Natural disaster such as flood, fire, electrical storm, earthquake, typhoon (hurricane) or tsunami (tidal wave).
Data recovery that has been attempted by an unqualified company.
What To Do Following A Crash(and what not to do!)
Choose a "Data Recovery" company very carefully. Sending your drive to an inexperienced company can only cause more damage. In fact, this is the most common cause of "data loss".
Do not remove the cover from the hard drive. This will only cause further damage.
Do not attempt to recover data with commonly available software utility programs.
Do not continue powering up a drive you believe has been damaged. It can and will only get worse.