I recently faced hard-disk failure [500GB, SATA - Seagate] in my laptop. I replaced the failed one and bought an enclosure for it. The enclosure is not recognised by any system. I am not sure if it's a mechanical failure because I can hear the disc spinning when I connect it to the USB port.
What can I do to recover the data from my hard drive?
If you have an available desktop system, take the enclosure out of the equation by attaching the drive in question directly to power and an sata port on the desktop. If the drive is seen in the BIOS, and in the Windows device manager, there are all kinds of utilities to try to recover the files from the drive. If it's still not seen, then you have a larger problem.
The enclosure will have a USB-SATA bridge board. How the bridge IC identifies itself to your PC host will depend on its firmware implementation. Some USB bridge ICs will not enumerate if they cannot detect a hard drive, while others will identify themselves whether or not a HDD is present.