What would be the reason for my toenails to be turning black. They are also becoming hollow underneath. What should I do?

Toenails. You probably have a fungus in your toemails. Many therapies are out there but soaking them in vineger twice a day and using some tinactin (tolnaftate) spray may help.
Onychomycosis. Depending on your age it could be fungus on the toenail. You should see your doctor who could get a scraping and send it to the lab to be analyzed.
Fungus. Is the most likely cause. I suggest you see a podiatrist.