In cases where the nail lifts off the nail bed, it is likely the toenail will fall off. But not every black toenail separates from the nail bed. If you notice bruising under your nails but the nail is still flat, take preventative measures to keep your black toenail from sustaining further damage that can lead to it falling off. Don't fret if your toenail does fall off, however. Most nails regrow over the course of several months, and your running routine can resume as soon as it is comfortable.