Back in december 2020, i had a flu. since then, i feel i have completely lost my senses of smell and taste. i just want to know what actually causes this type of after-effects and what should i do to repossess my senses?(asked on behalf of my sister?

Following a "simple" flu, it is unusual to suffer such symptoms unless you had significant nose and sinus problems BEFORE the flu. If this is the case, she should see an ENT surgeon. If this is NOT the case, her "flu" may have actually been COVID-19 infection, which causes such lingering symptoms. She can get tested for antibodies to the virus, which can tell if she had that infection.