Eyelashes fall out normally from time to time, just as hair elsewhere on the body does. But they can also fall out for other reasons. The medical name for this problem is madarosis, and some mechanical causes are pretty obvious - such as the way you remove eye makeup. Wiping your eyes too vigorously can pull out a few lashes: better to use an eye makeup remover that does the job without the need for rubbing. In addition, heavy mascara can actually weigh down delicate lashes and cause them to fall out, and the pressure of an eyelash curler can pull them out as well.
Other potential causes include eyelid infections, which can result in the loss of eyelashes. An undiagnosed thyroid condition (either underactive or overactive) can also be to blame. (If so, your eyelashes will stop falling out once the thyroid problem is treated.) You also should investigate the possibility of infection with a mite called D. folliculorum, which can cause swelling and inflammation of the eyelash follicles. If you're losing hair elsewhere on your body, alopecia areata, an autoimmune condition, may be the problem.
If none of these possibilities seem to fit your case, you might consult an eyelid specialist (an oculoplastic surgeon), who should be able to figure out what's going on. A drug called Xalatan used to treat glaucoma reportedly can prompt eyelash regrowth. However, this drug can cause the iris (the colored part of your eye) to change color, and it has a number of side effects including blurred vision, burning or stinging eyes, and skin rashes.
If no medical or mechanical (you're not absent-mindedly pulling out your lashes, are you?) cause can be found, you can always resort to false eyelashes if you feel self-conscious.
Andrew Weil, M.D.