Has the drive been working when you plug it into your laptop? Does it work if you plug the drive into your desktop computer?
If you have never plugged it into your laptop and it did not work on the laptop, you laptop may not be providing enough power through the USB to run the external hard drive. In this case you need to plug the external drive into the mains (does the external drive come with an AC adaptor?)
If you external hard drive has been working when plugged into your laptop, desktop or any other computers, and you have it for a few years, it is possible your HD is screwed.