Your eyelashes may fall out for a variety of reasons. The most frequent of these is from rubbing or trauma. This may occur when you are stressed and not even aware of touching your lashes. Also, overly groomed lashes, such as from curling, crimping, or heavy mascara may cause them to fall out. If this is the case, they should regrow with conservative treatment and by minimizing manipulation of the lashes.
Other less frequent causes of eyelash loss are alopecia areata, and scarring alopecias such as frontal fibrosing alopecia or lichen planopilaris. They would likely by accompanied by hair loss elsewhere on the body, or other symptoms such as redness, flaking, or scale. If you have any other these, you should consult with your dermatologist.