Ouch! You either have a dead short, or a bad breaker. Check the recepticles on the line that is tripped. You may have smoked something on one of the recepticles and it is shorting out. If this is the case, you may see burned or charred material on the recepticle.
You can check to make sure the breaker is ok by removing the wire attached to it and then try to reset the breaker. If the breaker holds, you will have to check all the recepticles/appliances associated with that circuit by removing the covers and checking to make sure the hot wire is not touching the ground or neutral in those recepticles. Leave the wire off the breaker when you do this.
If you are uncomfortable doing this, call in a pro.