|Probably (you guessed it...) a faulty hard drive.|
I'm guessing it's a 3.5" desktop-type HDD, not a laptop or portable external drive?
Any chance of plugging it into an external caddy and see if it spins up and is recognizes by another PC? That way you'll know for definite where the fault lies.
It's most likely to be the HDD itself. If so there's a chance you've lost all the data on it, (unless you already backed it up), or else opt for an expensive, specialist data recovery company to take a look at it.
You could always try the old freezer trick. Sounds daft, but if it's definitely the drive, what's to lose by it?