Your fiance needs to see an ear nose and throat specialist ASAP, like right now.
There are several possibilities as to the cause, but sudden hearing loss like that is an emergency situation that needs to be seen by a doctor that specializes in hearing loss.
There is a viral cause to this, that needs treatment with steroids immediately to save the hearing and if treatment is not started right away, the hearing loss can be permanent.
What isn't normal is to have a sudden hearing loss like she has.
Please get her to an ENT right away.
Fran