I am 20years old boy. beard on my face was growing properly, but now it is not growing properly.

Properly how?: I dont understand it sounds like you were developing a beard at a normal puberty age and now the bears stopped growing? If it has ceased growing do you have normal growth in the pubic area, underarms , extremities? If not see an endocrinologist. It would be rare to suddenly lose your testosterone level but smile tests would show if you have normal testosterone levels