if you locked it using tools from a floppy disk, it should have saved a log file for you. Check for one and it will tell you what password you locked it with.
If not..well, you have 2 options I can think of:
1) trial and error. you can attempt to unlock the hard drive I think 3 times without cutting power off to it. If you think you know where you messed up typing, try a couple things out and use atapw (sp?) to see if it unlocked or not. Once you attempt it 3 times turn off your computer for a second, turn it back on, and try again...
2)even if your hard drive is a doorstop, your xbox isn't necessarily. Buy a modchip and a new hard drive, use xboxhdm to format the new drive and load the slayers system files onto it, and you have yourself a usable xbox again. Just make sure you flash a BIOS onto your chip that does not require the hard drive to be locked.
If neither of these are an option, I would try to google methods of unlocking hard drives of your brand. I believe someone found a way to unlock the seagate drive when he accidentilly locked it with the wrong pw. search around, good luck!