Note these instructions only apply to Mac OS X 10.2 and above. Footnotes will be marked with a parenthesis like (1). The ideal solution would be simply to recover from your backup rather than go through this lengthy process. But if you have no backup, you have a few options here before taking it to a data recovery specialist.
If you are greeted with a flashing ? folder, or a do not enter sign upon startup and
resetting the PRAM doesn't solve your problem, and following the directions on Knowledgebase article 106214 is not able to repair your drive, you may need to take a couple extra steps to repair your drive. You will need the following:
1. The operating system disks which were used to install on the hard drive which can't be repaired.
2. An external Firewire hard drive (1) (A USB hard drive won't work on a PowerPC Mac to do this job) at least 10 GB larger than the drive you are attempting to repair.
3. Prosoft Data Rescue II (1)
4. Alsoft Disk Warrior. (1)
Once you have these materials, create a bootable external hard drive by booting into the installer disk from step 1 with the startup manager.
Then format the external hard drive after attaching it with GUID (2) partitioning using Disk Utility from the Utilities menu (3) of the installer disk which came with your Mac that appears after you select the installer. You will find a Partition tab with options how to partition. Choose the GUID partition method after selecting the external hard drive, and before erasing the external hard drive, and partition it for two partitions (one for at least 10 GB, one for at least the size of the internal hard drive). Be sure also to use HFS+ or HFS+ Journaled formatting. Once that is done, restart the computer from the installer disk again, and install the operating system on the external hard drive's 10 GB partition customizing the install to only install the basic operating system and nothing else. I would put both a copy of Prosoft Data Rescue and Disk Warrior on the 10 GB partition. Once that is done, attempt recovering your internal drive to the larger partition with Prosoft Data Rescue. If you are able to get usable data or already have a backup, then go ahead and try to run Disk Warrior on the internal drive.
If Disk Warrior can't fix it, you may be due for a new drive, and if any data is not restored, a data recovery specialist will be needed.
(1) The utilities and drive should be known to be compatible with the operating system version you are trying to recover. Check with the vendor to make sure the exact versions are supported.
(2) GUID is for Intel Macs only. Use Apple Partition Map, also found in Disk Utility in the Partition section if you are recovering from a PowerPC Mac.
(3) Disk Utility is found in the Installer menu of the Mac OS X installer on 10.3.9 and earlier installer disks.
If you have further questions about what any of these terms mean, or need advice about which drives to buy, or how to backup, just post to Discussions and ask the question.