Even if the alternator could not keep up with the power demands of the plow work and started using battery, when done with the plow, the voltage should come right back up to 14 volts.
Jump the regulator to be sure that's not failed. if the alternator has a ground regulated side - then ground that terminal or wire from the tieminal/
Regardless, recharge the battery using a slow charger. Otherise there's a good chance of cooking something when you do get the alternator going again.