One possibilities is damage to the nasal lining and cells.
An ear nose and throat specialist can determine if this is the case.
Allergies, nasal polyps, etc.
Other possibilities have to do with how the brain processes smell: this can happen with seizure disorders [some would not have other symptoms], brain tumors, etc.
This is a reason to see your doctor, maybe an ENT doctor and maybe a neurologist.
Your primary care doctor can examine you and determine the direction.
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