Often when a breaker trips, in order to reset it, you must firmly push the breaker all the way to the OFF position first (as far to the off position as you can get it to go- you will feel a bit of a resistance as it resets the tripping mechanism inside) then attempt to turn the breaker back to the ON position. It should hold now. If the breaker has a history of tripping, it could have gone bad (not very common though) in which you would want to have a qualified electrician replace it. For safety, You should always have an electrician try to find out what is making the breaker trip. Ignoring these types of conditions can lead to house fires.