Ok, one of three things possibly occured.
1: The firmware "forgot" what the boot drive is.
2: The hard drive is dead or some other hardware issue (disconnect other hardware and reboot)
3: Somehow software got messed up so now it's not considered bootable
1: Reboot the Mac holding the option key down, you hopefully should get a selection of bootable drives with a arrow underneath each one.
See the OS X boot drive? (pray here) now click it and it will boot, head to System Preferences and set the Startup disk to it once again, that will tell the firmware who's boss. Your done, count your blessing and make a backup of the files off the computer to a storage drive.
Most commonly used backup methods
Run through the relevent list of fixes here as best as you can (1-5)
#5 should see if the drive is dead or not as it won't appear on the left side.
Step by Step to fix your Mac
2: If you don't see anything holding the boot key down.
The drive could be dead, however, it might be the Mac, software or the cable, the drive itself, if removed from the Mac and a SATA to USB adapter used ($12-$30 online) it might work just fine on another Mac, or a Windows PC with MacDrive installed to get at the files. (filevaulted no way) you won't know this unless the drive is physically removed from the Mac.
If the drive is truely dead, then there are very expensive platter recovery services for a few grand will take it apart and image the drive. Drive Savers is one for instance.
3: If you don't see a bootable option holding the option key down,
that could mean it's just not bootable, the drive may still work and the data can be recovered. Sometimes something goes wrong with the retention of key info on the drive, so it appears not bootable.
For this you need to create this bootable external drive, then you can hopefully access the internal drive and recover files
Create a data recovery, undelete boot drive
Once you have gotten the files off then you need to perform this proceedure from the 10.6 disk on the entire internal drive.
How to erase and install Snow Leopard 10.6