It depends on the vehicle (shame on you for not posting that
) but most often its the turn signal return cam that is failing. If you pull the streering wheel you would see that the turn signal switch has a little mechanism that turns the signal off when the wheel turns back.
the parts are cheap, but the tools to do it yourself (steering wheel puller) might make it not worth it. I'm 34 and I have a car with the turn signal stalk hanging by a wire against my left leg, so I suggest (if you want to save money) that you continue operating the swtich manually.