could be an issue with the power supply, in which case the data could still be intact.  Otherwise you'll probably need to pay to have the data recovered. Actually you'll probably have to pay anyway to get the drive put in a new non-faulty case (unless you're confident you can do it yourself).

Good luck. Hard-drive failure sucks.

It's too late in your case, unfortunately, but there is simply no excuse for not having a backup strategy. To another computer, or a server in the 'cloud', or even your most important files to DVDs from time to time.

Your disk probably just needs a good fsck. Since it's external, there's a chance you unmounted it in a way it didn't like, ruining the journal (or something), and now the OS refuses to mount it, by design, in order to preserve the data on it so you can fix it. (But shouldn't the OS also try and fix it for you? I dunno...must be a Mac thing. Pay us money to fix it for you...!).

The operating system will tell you useful things as it does the things it does. Try going to a terminal and running this:

tail -f /var/log/dmesg

Then plug in the disk and look for anything odd as it tries to mount it. (dmesg is basically the log for drivers and one of the most useful tools to find hardware problems).

I say you stand a good shot at getting the filesystem back if you boot a Linux machine (say, a live CD), and making use of the fsck.hfsplus command. I'm sure there are guides to help you if you google.