It is not clear exactly what you mean by a fallen nail. If you mean that your nail has fallen off due to some injury, then there is not much that you can do or even need to do. Keep your toe and the entire foot clean and dry at all times to avoid infection. In addition, you can apply a standard ointment that is used for injuries, which you can get over the counter at any chemist. The nail will most probably grow back in a few weeks, once the injury has healed. If the nail matrix below the cuticle is itself injured, the new nail may be crooked or otherwise deformed. In some cases, the matrix may be too damaged to grow a new nail.
There are several other conditions that can also cause the nail to separate from the nail bed. One possible cause is a fungal infection, as a result of which the nail can become thick and opaque, and either crumble or fall off entirely. If the infection is mild, you may be able to treat it by keeping your feet dry and clean, and regularly soaking your feet in a solution of vinegar and water. However, there are several other infections that can have the same result, so if an injury is not the reason your nail has fallen off, you should visit a doctor.
answered by G R