How do I re-attach a crank that has fallen off? I have already read My crank fell off, how do I re-attach it?, but it did not answer my question. 

I was biking up a hill in first gear, and I felt my left crank was a bit loose. Never having had problems with my crank before, I kept going. Sure enough, the left crank fell off. 

It appears there is a square hole in the crank which the axle goes through (which is also square). This is held on by a hex bolt about half an inch long. The bolt says "CH 12.9" on the head. 



Below is a picture of the crank where it attaches to the axle. Note the threads. There is supposed to be a circular piece that goes into them and holds in the bolt. Unfortunately, it must have fallen out before the crank fell off and is probably in a ditch who knows where now.



However, I believe damage has also been done to the crank. I'm not sure if you can tell from this image, but there are dents in the hole, probably from the axle. I have tried to put the crank back on the axle, but I find it incredibly difficult. I decided against a hammer with a wood block because I'm not sure if that'll make it worse.



That's my problem. Here's my question:

Is this repairable? If so, how do I go about it?
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.)