Circuit breakers are made in several different ways. Some breakers are designed so that when they trip there is a red or orange flag that pops into a window to show that it has tripped. Breakers have three positions: on, off and tripped. Tripped position can be either halfway between on and off or even look like it's still on. The "visi-trip" style breakers made for Square D's QO style panels has the red flag window feature and is sold for a little more than their Homeline stye breakers without visi-trip. The two styles of breakers do exactly the same job and are internally similar. However, the visi-trip breakers offer the bonus of seeing that the breaker is tripped by a simple visual inspection.
Push the breaker switch all the way to the off position until the red flag disappears. Breakers are spring loaded on the inside and in order to reset them the switch must be physically pushed all the way to the off position.
Turn the breaker switch back to the on position. If the breaker trips and the red flag is showing again, with a straight tipped screwdriver loosen the screw holding the black wire into the breaker. Pull the wire out of the breaker completely.
Reset the breaker again. Push the switch all the way to the off position then turn the switch back on. If the breaker still trips without the black load wire connected, the breaker is bad and you should replace it with a new one of the same size, brand, and style.
Unplug everything from the dead outlets on the circuit that is tripped if the breaker resets and doesn't trip with the black wire disconnected. The problem is probably something plugged into an outlet that is shorted out and tripping the circuit.
Reconnect the black wire to the breakers screw and tighten the screw with your screwdriver. Reset the breaker. Push the switch all the way to the off position and turn the switch back on. If everything is unplugged from the outlets that were dead and the breaker still won't turn on, consult an electrician to find the short in your electrical circuit.