Sudden indentation of nails can be caused by nail fungus disease, trauma, psoriasis, lichen planus and other factors. 1. Nail fungus disease: nail fungus disease can be caused by fungal infection of the nail plate, resulting in the nail plate being attacked by the pathogenic bacteria, which can cause the appearance of depression, and there can also be a thickening of the nails, yellowing and so on. 2. Trauma: the nail has been impacted by external force, which can lead to changes in the shape of the nail plate and the appearance of depression. 3. Psoriasis: when psoriasis affects the nails, it can lead to “thimble-like” depression and other changes in the nails. 4. Lichen planus: when lichen planus affects the nails, it can lead to thickening or thinning of the nails, and the surface becomes uneven and rough. If your nails are suddenly sunken, you should seek medical advice and treatment under the guidance of your doctor.