How do I re-attach a crank that has fallen off?
First off, got to a bike shop and get a new crank bolt.  They're a fairly standard item.

Can't tell for sure if your bike had "self removing" bolts or standard ones, but it doesn't matter that much, a standard style should fit.  The large threads in the crank arm are for holding a removable trim plug over the bolt (or holding the "self-removing" part of a self-removing bolt) and for facilitating a crank remover (which is kind of not needed at this juncture).  You can ask for a new trim plug at your LBS, but it's not really needed.

Once you have the bolt, you can try reattaching the existing crank arm.  It will need to be really tight -- look up the torque specs on the web if you have a torque wrench, otherwise just get the bolt damn tight.

But once a bike has been ridden with a loose crank arm the arm is usually damaged beyond repair -- likely it will keep working itself loose, and will eventually damage the crank axle (if it hasn't already).  So you probably need to replace the arm, and you might as well replace the crank cartridge at the same time, since it's probably cheaper than the new arms.

(So I'm guessing you've figured out by now that you should never ride a bike with a loose crank arm.)