Thank you for visiting the EmpowHER community and for sharing your question about your nails turning black.
If you haven't injured your nails, then most likely you have a fungal infection and you should see your doctor.
The fungus could also spread to other nails or your skin. Over time, the infection can cause permanent damage to your nail or nail bed so I wouldn't put off going to the doctor for a very long period of time.
Please let us know what the doctor says.