The blower motor in my car doesn't always come on. When it does come on it works great; however, it doesen't alwsys work. What should I do to fix it?

Sometimes The blower motor which pushes air out of my vents doesn't always come on. The ac and the heat work fine. It will swith from vent to floor to defrost or a combo just fine. It just doesn't always kick on. Occassionally if it is not on and I hit a good size bump it will kick on. What should I do to fix this problem?