Don't panic (yet) Most likely the system is corrupted and won't boot the CPU.
Try booting from the hardware test CD or the install CD and see if the CPU starts up.
If it Does, do NOT reformat the drive. Run Disk First Aid and see if it fixes the drive. Restart again and see if the CPU boots after repair.
If DFA doesn't see any drives, you are in trouble.
Make a decision NOW about your data. Just how important is it?
Do you have any backup? Sending the drive out a recovery service is expensive ?$1000 or more, but you WILL get the data back almost certainly. There are other things you can try on your own to get the data back, but every attempt you make will REDUCE the chances of by professionals.