External hard drive not recognized
You need to first narrow down the problem to either your PC or the external hard drive. Do you have access to any other PCs you could try connecting your hard drive to? (think friends, family, public PCs, etc)
If the hard drive works on other PCs, we know it isn't a problem with the hard drive.
If it turns out that the hard drive is fine and the PC is the problem, I would suggest going in to Device Manager and uninstalling everything USB related. Once you have done this, either reboot your PC or right-click inside device manager and "scan for new hardware". All of your USB related devices should reload their drivers automatically. You may want to try this both with and without the usb drive connected.
Another tip would be to try various USB ports.