Has the crank retaining nut come off, in which case the crank arm will eventually come loose, your local bike shop will be able to supply one for minimal cost, or just the dust cap that pushes onto the crank. Again your bike shop may be able to help, or you can go without one.
Saying that the crank will eventually come loose seems like the logical thing to happen, but logic and obvious isn't always that obvious with these things. The crank will most likely not come loose ever. I only say most likely because I don't know how well it was torqued to begin with. But...in general if a bolt or nut falls of, it came loose because the crank squirmed itslef further up the BB spindle, leaving the bolt/nut behind, which obviously cannot tighten itself.
To visualise this phenomena, imagine the sqauare taper of the inside of the crank to be four brushes with short bristles, each arranged 90 degrees from the ones adjacent to it. In other words, a square hole lined with bristles. Now, force (torque) that bristely hole up the square taper BB spindloe and imagine the orientation of the brush fibres. If you imagined right (remember, this is just an analogy) the you'll see the bristles bent and facing towards the nut/bolt end of the square tab. Now wind a nut onto that square tape and keep that square brush in its place. Bristles still bent towards the threads.
Now...force-turn the crank and look what happens to the bristles. Those on the leading flats are forced flatter and those on trailing flats are relaxed enough to shoot back and make themselves straight. If you can't visualise this, think of a scribbing brush pushed forcefully along a length of very coarse sandpapere. You can see the bristles bend backwards with respect to the line of travel. Now lift the (imaginary) brush and you'll see the bristles straighen out and reposition themselves upstream.
Upstream on the BB spindle translates to is further up the taper. In other words, a torqued crank eye moves upwards on the BB spindle leaving the nut loose in its wake.
The nut diodn't fall off because it worked loose. The nut stayed in position but the crank moved up the taper leaving a gap between it and the nut and now the nut can fall off. Hence, the importance of a plug in that hole. That prevents the nut from falling off and getting lost.
