A hard drive crash without warning is sometimes better than one with a warning. If the surface is failing in some way, you'll often hear a grating noise from the head as it repeatedly resets itself trying to find the data. If it's been silent, and the computer won't boot then hopefully it's simply a soft error that (if you're using Windoze) CHKDSK will find and fix (as others have said). There are a number of ways of running CHKDSK from a computer that won't boot, but the easiest is probably booting from the original installation CD that you may have had. If you don't have one, then my next step would be to install it as a slave in another machine (as someone else said!).
The next two really are for the cases when nothing can be done:
If it's genuinely stuffed and the head is clattering around, you could try the remedy of desparation and pop the drive in a sealed polythene bag in the fridge. Give it an hour or two to cool down and then install it as fast as possible (eg connect the cables only and get to work. You can sometimes get five or ten minutes to grab the data.
Finally, check that the drive is rotating. Drives usually hum/vibrate slightly when they're spinning. Sometimes the grease on the spindle goes hard and the disc won't spin-up. (This usually only occurs when the machine's been off for some time.) I have been fortunate in the past to use a fast wrist spin and get one to rotate again so that I could grab the data off it.
Good Luck!