Power Jacks also called AC Jacks can fail for a number of reasons:
Solder Wear & Tear: Repeatedly plugging the power chord in and out can break the soldered connection on the motherboard. If this occurs, the solder can be re-flowed or completely resoldered to restore a solid connection.
Wire Wear & Tear: Some laptops have a wire that connects from the power jack to the motherboard. This wire can become frayed over time due to the movement of the power cord.
Fall Damage: Even if a computer was dropped and the area around the jack was damage it can usually be fixed. Sometimes epoxy can be used on damaged areas so that the jack is properly seated.