Hard drive is not recognized by my computer.
This generally means you have a hardware failure in the drive or enclosure. The simplest way to test which is to have a hardware tech move the drive into a new enclosure and see if it works.
This will void your warranty if you do it, so, if the drive is under warranty and there are no files you consider valuable on the drive, call the manufacturer for warranty support.
If you value the data more than the drive is worth then take it to your local hardware vendor and ask them to test the hard drive in a new enclosure. Good odds it will work this way but if not you may need the help of a data recovery specialist.
In North America I use Armor-IT, if you are in a different region and the new enclosure fails to garner results, comment back with your region(Country, State, City) and I will supply you with the closest reputable facility.