Individual eyelashes fall out all the time. People who wear contact lenses know this fact all too well. Eyelashes take about four to eight weeks to grow back. These tiny hairs (with help from their cousins to the north) protect your eyes from dust and sweat -- you'd definitely notice a difference if you were to lose them.
According to Yahoo! Health, the medical term for the severe loss of eyelashes is madarosis. People who suffer from hypothyroidism (an underperforming thyroid gland) often lose their eyelashes in small clumps. Damaging an eyelash follicle, or the small cavity that grows the hair, can also affect future eyelash growth.