Why haven't my eyelashes grown back?

A section of my right eye's eyelashes (probably about five milimeters) have fallen out all at once. I've been waiting for about two to three weeks and not even a little bit have grown back. It's still completely bald and it's hideous. I put petroleum jelly to help them grow back, but it hasn't helped. I use mascara, but I only put one layer on after they fell out. Please help me, it's starting to worry me.