collision detection not working?

i am new to unity.i am creating a basic platform game. i have a character which can be moved using gui buttons. All i wanted is if a object falls on head of character i want the character to to detect whether a object is falling on a character.please help me. thank you in advance.

more info is needed. is this 3d or 2d project? if 2d, make sure you using 2d colliders riggers