Hi,
There are a number of things that might have gone wrong, some completely disasterous and some pretty easily fixed.
I had an Western Digital firewire drive that failed on me and all I had to do was rip thing thing apart, take out the off-the-shelf hard drive and plug it into a spare firewire bridge. You can get firewire bridges from most computer stores – basically they are sold as a build-it-yourself firewire drive with all the parts except the hard drive. The one thing you will want to make sure of is whether the drive is ATA or SATA. Probably the best approach here is to take the drive to a repair shop you trust and ask if they can place the drive in a new bridge just to see if it is the brige that died.
I have also had good results with data recovery programs. I hear disk warrior is very good. I had a Lacie 500 Gig Raid drive fail and the program saved almost all my data.
There are some cases where all is lost from the start, but in my experience, if you have a very sudden failure, it usually ends up being a fairly easy fix whereas when you see a slow progression of more and more minor problems until the whole system crokes, it is usually a problem that is harder to solve.
At any rate, what is important is to not panic and try drastic measures.
Good luck
Lars