I have plugged in my failed HDD to my booted up desktop via Vantec sata to usb adaptor. I am able to see the failed HDD under "Disk Management" only in Win8 and not in Win7. The status of the failed disk is healthy(GPT Prottective partition) in win 8 and not initialised in win 7). But the disk is not showing in "My Computer" (as it is so named in win7 or "Computer" as it is so named in win8). I tried running data recovery software such as EASEUS but it does not detect the failed HDD as well.
those free data recovery softwares (such as easeus) will generally only recover data that's been lost due to being deleted, formatted, or partition issues. they still require the hdd to be in good working order and accessible (which it doesn't sound like yours is). whereas if the hdd itself has failed then data recovery can get a lot more expensive and complex, something even a majority of well-versed techies couldn't do.
try plugging the hdd directly into the mobo and try other softwares, if it still isn't even detectable then i'm afraid i've got bad news for you: either take it to a data recovery specialist (open your wallet and cross your fingers), or say good-bye to your data.
good luck, i hope you manage to recover your data somehow.
but regardless of the outcome, learn this lesson: backup your data!