Toenails are already dead, so it is not quite right to say they have become that way. I think that a fungal infection is most likely the cause of this dead appearance. Your doctor can tell you if this is the case. Treatments are available in the form of topical creams or oral medicine.
If you prefer something of a home cure, try some Vicks VapoRub! I have heard of this method for years and years. It may take a long time to work, but so do the other cures. Also, keep feet dry, wear clean socks daily (white ones can be bleached), and expose feet to open air when possible (sandals, flip flops).