Losing an earring back is annoying enough, but if you have no idea where it went, you're stuck. If you have a pencil handy, you have an easy way to put your earring back in — at least until you can find a replacement earring back.
Cut a few centimeters off the end of the pencil eraser, then stick your earring through it to make a hole. Make the hole before you try to use it as a temporary back so you'll have an easier time putting it on. Once you get home, you can swap the eraser out for a back from another earring, or just buy a replacement back.