Normally can lights are attached to the joists, so the housing itself wouldn't usually fall out. What is probably falling out is the trim piece that fits inside the housing.
If you remove the bulb, you can usually pull the trim piece out partially. Once you do that, you'll see a couple wires (look like coat hanger wires). If you squeeze those you can pull the trim piece out the rest of the way. Those wires are what hold the trim piece in the light housing. Over time they can lose their "springyness" and slide out a bit.
Just take the trim piece with you to a home store and you should be able to buy a replacement, or you might be able to bend the wires a bit to get them to fit in snugly again.