The voltage regulator is an electronic device that prevents excessive alternator voltage. It senses the amount of voltage and then adjust it by inserting or removing a resistance in, or opening and closing, the field circuit in effort to maintain the ideal amount of voltage needed.
If there is a problem with the voltage regulator, higher or lower voltage levels can go from the alternator to the battery. If the regulator fails completely, it can cause a break in the circuit and there will be no electrical connection from the alternator to the battery, and vice versa. The above mentioned failures could result in the electrical components not working and the car not starting. If the voltage regulator is located on the alternator, it can be replaced separately. If it is located in the alternator, then the alternator will need replacing.
Most manufacturer factory warranties will cover the voltage regulator under the bumper-to-bumper warranty. For extended coverage, an extended auto warranty is an option to extend coverage of the voltage regulator beyond the expiration of the factory warranty.