1. Touch the drive (or listen to it) to feel whether it's spinning. Some drives gradually suffer from spin-up problem but otherwise work fine once spinning. If it doesn't spin at power up, gently knock on the side the drive once or twice to jump start it. This works best if you knock on the drive approx. one or two seconds after power is applied. Repeat the procedure a few times and add a little more force if necessary. Remember that too much force can permanently damage the drive, but again, you have nothing too lose at this point.
2. If drive spins normally and stays spinning, try listening for irregular sounds emitting from the drive. A series of 'clicking' sound usually signifies multiple bad sectors including the boot sector that can prevent drive from booting. If drive 'Auto Detect' is enabled, make sure that its signature is shown at boot screen. If not, drive is certainly suffered from major hardware failure.
3. Check system's CPU to make sure it's not overheating (CPU can run warm, but should not be hot) due to a failed cooling fan, etc. Overheating the CPU can cause the system to be unbootable or cause the system to reboot itself frequently.
4. You could use another system to test the problematic drive to make sure that the controller is not at fault. Try both "Auto" and "User Type" (where you manually enter the drive's parameters) settings.
5. Try booting with a floppy and run 'fdisk' to view drive information. Some drives suddenly lost all of their data possibly due to corrupted FAT, but otherwise, continue to work fine o