Step 1: Identifying the problem
Is it hardware or software?
Powers on but BIOS doesn't see it correctly (size and partition) DOS tools don't work.Windows doesn't work
Software. Go to the Hard drive manufacturer site and download and make their boot floppy or boot CD.
Use their software tools. For example low level format the drive, which takes a few hours but makes the drive workable again. The drive is empty but not broken. FDISK format etc.
Doesn't power on.
Find broken part and replace it.
Difficulty is high as it's hard to tell what part is broken.
I accidentally broke my hard drive. I had my tower case on its side and powered on, the hard drive was on its component side down. The hard drive accidentally moved and short circuited on a mounting screw on the grounded frame of the case.I was lucky in the sense I could easily see the broken part. A transisitor on the circuit board broke/exploded/popped in a second.Hopefully this was the only broken part.