I would first try and install the drive by itself, in your computer -
- take it out of the enclosure and plug it into one of the SATA ports on your motherboard, and connect power to it
- then enter BIOS and see if the motherboard sees it
if it does, then boot into windows and see if you can access it
if it doesn't, then check to see if it's an incompatibility with your motherboard and possibly update your BIOS
if you have the latest BIOS, and it doesn't recognize it, then the drive is probably bad - connect it in another computer to see
about 3 years old and maybe once a week a on av.
I'd recommend following the suggestion by Dingo07. Also, if it turns out to be a bad drive, you could try putting the hard drive in an air-tight bag in a freezer to let it really cool off and after its very cold, try putting it back in your computer and see if you can get any data off of it. Believe it or not, this actually helps in many cases.
Considering that this back-up drive is three years old, it would have been a good idea to buy a back-up of your back-up about a year ago. Three years is a common lifetime for modern hard drives, especially external hard drives that tend to be poor quality drives. Has this drive ever gotten hot that you know of? Some external enclosures have insufficient cooling and it's usually a good idea to check and if needed, augment the cooling.