The eyelash is a kind of hair in people' body, so that it has its own lifecycle. However, the eyelash has one of the shortest hair lifecycle, which is between 3 to 5 months. After the period, it would be fall out with 1 to 3 hairs every time. It is the normal process of metabolism. Another situation is maybe you get blepharitis, which causes by bacterial infection of eyelash and the edge of the eye. You had better go to see eye doctor to check whether you get blepharitis, if your eyelashes are falling out too much once.
Compared with that of other body hair, the growth rate of eyelash is slowest. If one eyelash falls out, it will take about two months to grow out again. The main reason of eyelash falling out is that the area of eye is too dry. You may try to mix a little olive oil and red wine together, and daub the mixture on your base of the eyelash using cotton bar before you sleep every night. This way can help stimulating the growth of eyelash. By the way, if you use to make up, you'd better stop using mascara.