A remarkably common malady among humans.
Toenails, composed mostly of Keratin, can be lost in a number of ways including infection, trauma, toenail fungus, or amputation of the toe, foot, or leg.
Most everyone has lost or broken a toenail to the cutaneous material at some point in their lives with few complications and a short recovery span. In fact, toenails grow at 1/36th of an inch per day so the normal person can regrow a lost toenail within one month.
However, there is a limit to what the toenail can take with resiliency. Losing the forth metatarsel will always do it, but it is only the soft tissue damage that determines the loss.
In addition, many sorry humans are simply not born with one or more toenails. Life has simply dealt bad hands to these unfortunate souls. They live life on the margins, unable to wear steel-toed boots or operate a Swiss Army Knife with their toes.