May I suggest heavy drinking? It won't solve your problems, but it makes them less painful for a little while. Celebrate the fact that you have been very diligent about making backups.
This sounds like your master boot table, or similar construct decided to go bye-bye. It may be the result of a particularly nasty virus or worm - but that is only speculation.
Try to get a bootable CD loaded with some disk utilities that you can use to check out the HDD. I don't know which utilities, but you might be able to finds some good tools at majorgeek.com. It may be best to get a local geek friend to help you out on this.
I would be surprised if you were able to recover the drive with the data in-tact. Be warned that you may have just lost everything and will need to reinstall the OS and all of your applications.
If all else fails - try 1/2 pint of Harp and 1/2 pint of Guiness - lather - rince - repeat.
Good luck - I think you might need it.