It might be possible that your hard drive has suffered some sort of mechanical failure...
Do you mean that the drive is no longer recognizable by the BIOS?
Were there any strange or unusual noises coming from the drive when you last attempted a boot sequence?
These are certain signs of HDD failure in which case you shall need the help of professional data recovery services.
I do know of a very unorthadox method to which i have used to certain degrees of success in the field, but i am loath to explain this method for fear of the criticism it tends to attract!
Impact IT Solutions (Sydney) come highly recommended, however im sure that there are many reputable services in your local area!
They provide free quotes and work on a...No data, No charge policy.
http://www.impactit.com.au/datarecovery.htm
Cheers