Post #3 in http://forums.cnet.com/5208-7588_102-0.html?threadID=354911 gives a useful list of DIY data recovery tools.

However, the disk should be alive to a certain extent for them to work. I assume you already tried to access it on another PC (just in case your USB is broken). That leaves the 2 essential components of the disk:
1. The electronics in the enclosure.
2. The disc with its SATA or IDE connection.

If #1 is broken, #2 will work in a new enclosure, or connected via a USB to SATA/IDE-cable.
If #2 is broken, however, you'll need a professional data recovery company and a lot of $$$. Try drivesavers.com or krollontrack.com.

Only trying will tell how bad your situation is.

One other point: an external drive is unreliable. Much more so than an internal drive. You really need to be prepared for its failure. And then its not dramatic at all. All you've got to do: order a new one and get the data back from the backup you made. Not making a backup on another medium means the data that you are accepting the possible loss.

Kees