This is most likely due to a nutrient deficiency. Nails provide us with a great deal of information on nutrient levels, and absorption capabilities. The 'half moon' is called the lunula. Vitamin B12 and iron deficiencies are common in those whose lunula is reduced, or absent altogether.
Iron levels can be determined in a standard blood draw. However, please note that testing B12 levels in a standard serum blood draw does not always accurately determine intracellular levels of B12, thus these deficiencies are often missed. Specific tests called Methylmalonic Acid (MMA) and RDW can be much more helpful in determining true intracellular deficiencies.
If deficient, consider 1000-5000 mcg of Vit B12 daily. As B12 deficiencies can often signify a deeper issue involving malabsorption in the GI tract, be sure to work with a functional medicine expert for appropriate testing and interpretation. You also may want to consider vitamin infusions (IV's) to build up your nutrient levels while working on the root cause of the deficiency.

Dr. John Dempster, ND
www.thedempsterclinic.com