As Loukas stated take the HD out and put it into an external case.
It's best to get a combo USB & Firewire case. I use Micro Center my self but any good computer store out there that have them as well.
Here's the link to the Micro Center cases page"
Before you close the case up check to see if you hear the HD making any noise.
If the drive is not making any noise you may have a stuck spindle motor.
Carefully bang the case squarely on a firm surface from a few inches once or twice (Platter parallel to the surface). If your lucky it may free up the motor. Don't go crazy here! as you don't want to kill the drive only free up the stuck motor!
The last option if the data is that important to you is to send the drive in to a data recovery service. Fair warning! this can be very expensive and they may not be able to save your data if the heads have crashed into the platters.