Just because a toenail doesn't look normal, doesn't mean you will lose the nail. There are many reasons the nails may not look normal including injury which can cause blackening of the nail, fungus toenails, ingrown toenails and even some diseases may cause different appearances to the nail. Whatever the cause of the discoloration, the question is always whether you will lose the nail or not.
Black Toe Nail: Usually indicates trauma to the nail (dropping something on it, runner's toenail, or bumping it). Depending on the extent of the injury, you may lose the toenail. Often in this case, the nail will stay intact and be discolored for about 1 year (the amount of time to grow the nail out).
Fungus Toenail: Otherwise called onychomycosis. Although this is a thickening and discoloration of the nail. Unless the nail is pulled off by catching it on something, the nail is usually not lost.
Ingrown Toenail: Unless the nail is severely infected and fluid builds up under the nail, only the portion of the nail treated will be lost. The nail usually is not.
Other: Most other discolorations of the nail will not usually cause the nail to be lost.