If I have trouble hearing in one ear, is it dangerous?

It can be.

Sudden deafness is fairly common, and most of the time, it’s a benign problem caused by the build-up of wax in the ear canal, water in the ear, or congestion from a head cold or allergies. In these circumstances, the nerves in the inner ear are working fine, but sound doesn’t get through, a situation called “conductive” hearing loss, said Dr. Steven D. Rauch, an otolaryngologist at the Massachusetts Eye and Ear Infirmary. Usually these problems resolve themselves on their own.