Drive symptoms
Blue screen/Boot failure
If your computer will not finish booting into Windows, your partition may be corrupt or your drive may be failing. If Windows at least starts to boot, then your data is in all likelihood recoverable. This type of failure is usually Level 1, but it may be Level 3. If this matches your symptoms, immediately turn your computer off and send your hard drive in for recovery, as further use can seriously reduce your chances for recovery.
Clicking
Clicking accompanied by improper operation is a sign that your drive is experiencing a hardware failure. Turn your computer off immediately. This type of failure degenerates quickly. Your drive has probably experienced a Level 2 or Level 3 failure.
Scraping
Scraping means that your drive has experienced a Level 3 failure, or what is known as a head crash. Turn your computer off immediately. This type of failure requires clean room recovery to replace malfunctioning parts, and is usually only partially recoverable.
Drive letter inaccessible
If your drive is no longer visible in My Computer, chances are good that your partition is corrupt and requires Level 1 recovery. However, the failure could be more serious. Discontinue use of the drive to ensure the best chance of recovery.
Drive not recognized in BIOS
When a hard drive is not recognized by the BIOS, your drive has probably experienced a Level 2 or greater failure. This requires clean room service and replacement parts.
Note: Dying drives have only so much life left. Powering on a hard drive with hardware problems can do further damage and make your data permanently inaccessible.