Not a medico so the story is that of the end user only.
I had a sudden onset loss of similar magnitude in one ear several years ago. Guessed it was time to check the need for aids. When the audiogram was run and the magnitude detected, it was off to the ENT folk (interesting to watch the boro peer into the eustachian on the video), then to imaging to check for an acoustic neuroma. With that sorted, it was put down to a viral middle ear infection and a prognosis of "tough luck". You may like to have a read through the following article
Viral Causes of Hearing Loss: A Review for Hearing Health Professionals - PMC (nih.gov)
Message is - sudden hearing loss is an emergency as some situations can be salvaged.