What Causes Severe Hair Fall At The Age Of 20?

hi doctor i am 20 years old i am facing problem of hair fall , my hairs from forehead has almost gone i look very ugly because of that , all my friends tease me because of that, every one notices that and i am very depressed because of that please doctor is their any remidy to grow my hair back please doctor help me, i am only 20 please get me rid of this problem