Contrary to myth, eyelashes rarely fall behind the eyeball. A layer of muscle and tissue blocks the front half of the eye from the back, and only with a tear in this lining due to severe trauma can this layer break. In fact, it's very rare for something to actually get stuck in the eye. The eye socket is REALLY good at removing waste fairly quickly.
In fact, that liquid layer in the eye actually prevents dust and particles from coming into contact with the actual surface of the eyeball. When you have the feeling that something is stuck in your eye, what you normally feel is the point where something comes into contact with the eyeball and causes a slight bruise on its delicate nerves, making it look like there is something in the eye. It's usually no reason to start freaking out because the tear duct and nasal passage are connected, so the eyelashes will be removed from the system through the nose. Don't panic, there are several things you can try to quickly remove an unruly eyelash from your eye.
A lost eyelash that reaches your eye is very irritating, especially if you can't remove it. You should not rub your eye, as eyelashes can cause a corneal abrasion, which will take several days to heal.