If you have valid credentials for the drive (user's credentials), you may be able to access the drive using the Access Utility iso. When you boot to the CD/DVD, it would present its own pre-boot authentiucation, and if you can authenticate it should bring up a command window, which you could use to copy the data from the drive.
Otherwise, you would need the client administrator credentials, then you could burn a recovery iso and boot to it to decrypt the drive. Without proper credentials, there is nothing else you can do to retrieve the data.
It may be possible for a third-party drive recovery service to get access to the drive, but these are generally expensive, and there is no guarantee that it would work. If the data is that important, you may want to look into that, and start calling around.
Check here for access and recovery processes:
http://www.symantec.com/docs/TECH171551