This seems to be a very simple thing called as disorder of taste and smell which can normally be over with time but in many cases it stays and then it is a big problem. Even the good doctors may not be able to diagnose it because there are so many reasons and underlying conditions which can create these problems in our body.
These disorders manifest in a variety of ways like partial or full loss of sense of smell, partial or full loss of sense of taste, getting a bad odor or taste from something that is normally tastes or smells good.
Doctors believe that these type of disorders are generally related to the conditions like Diabetes, Obesity, BP, Nervous system, Poor nutrition, Parkinson, Alzheimer, Multiple sclerosis, cold or flu, sinus infection, allergies, Head injury, Hormone changes, Dental or mouth problems, Nasal polyps, Exposure to certain chemicals, Certain medicines, Exposure to radiation therapy, Cocaine addiction, and Cigarette smoking. The list is very long and that is what I told in the beginning that diagnosing the problem is the main challenge for the doctor.
A doctor will go through the medical history of the patient and prescribe some tests to ascertain the status of the smell and taste. Some of these tests are comparing tastes and smells of different chemicals, measuring the lowest strength of a chemical that a person can sense, "Scratch and Sniff" tests, "Sip, spit, and rinse" tests etc.
Finally doctors will try to treat it using the standard procedures including the option of surgery if there is blockage somewhere creating the problem.
Consulting an expert ENT doctor can throw more light on the issue and treatment options.
Knowledge is power.