My crank fell off, how do I re-attach it? I was out riding on my mountain bike when my crank unexpectedly fell off (video).

How do I reattach the crank? There's a torque range specified (25-30 Ft-Lbs):

 

how critical is this? I don't own a torque wrench, if I use an allen key and just guess am I likely to either apply to much force and break something or apply to little force and have the crank work loose again?
In general, crank fixing bolts need A LOT of torque.  This chart list torques for several varieties of crank arms.  (I suspect you have a variety of Octalink.)  You're probably looking at something in the neighborhood of 350 inch pounds, which would be 35 pounds applied to a 10-inch lever arm.

Too much force and you shear the bolt.  Too little force and the arm works loose again.

The torque spec embossed on the bottom bracket cup is most certainly for the cup, not the crank arm.

Unfortunately, if your crank arm fell off I suspect it's been damaged, and is apt to no longer fit tightly, no matter how tight the bolt.