Sounds like the battery is dead. The voltage regulator controls recharging the battery while you're driving.
Turn your head lights on and see what happens to them when you try to crank the engine, then holler back. You can also use an inexpensive digital voltmeter to measure the battery voltage. A fully charged battery will read near 12.6 volts. A good but discharged battery will read near 12.0 volts. If you find closer to 11.0 volts or less, suspect a shorted cell in the battery.