What Causes Big Toe To Turn Black And Its Nail To Come Off?

Hi! A few months ago, my big toe turned black. I really don't know what caused it other than I think it was bruised from my toenails being too long with tennis shoes. Anyway, now almost my entire toenail has come off. It doesn't hurt anymore, but I'm not sure if I need to see a doctor. Will the remaining nail fall off too?