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

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.

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.

Fungus: Is the most likely cause. I suggest you see a podiatrist.