OS X partition doesn't show up after dropping iMac, Windows still works
If you are running 10.6.5, try booting from the original installation disk, then open disk utility. 

Run a "Repair Disk" both on the whole HDD to see if there is an issue with the partition table and on the boot volume that isn't working to see if there is an issue with the file system. If this disk is not repairable, you may have damaged the HDD with the drop, in which case not much can be done in terms of troubleshooting. You have a local backup, so your data is at least safe. 

Alternatively, if you have another Mac running OS X, you can attempt to mount the disk using Target Disk Mode. This will essentially treat the internal drive on the iMac as an external drive on the other machine. Then you can run the troubleshooting steps above in a similar manner.