Nails grow from the pocket beneath the skin “matrix” that produces new cells. The old cells are pushed by the new ones to get packed together. After being forced, the old ones become hard and compacted, and then, they are moved up to the surface of the skin. If you lose your whole fingernail due to an illness or any other reason, it will grow back within 4 to 6 months only if the matrix is not damaged. If your nail is not growing back, it is a matter of concern. You should immediately speak about it to your doctor to know why your nail is not growing back.