Do you have an expansion tank before the water heater? If not, add one. If you do, check to see if it is not shot. The tank should be nearly full of air, so if you tap it and it sounds like it's full of water, it's probably shot. If you take the cap off the bottom and check it with a tire pressure gauge, if you get water out, it is shot.
If you have one, and it doesn't shoot water out, turn the water off to the house, then open a faucet to relieve the pressure, then using the tire pressure gauge, check the pressure in the expansion tank...it should be set to approximately the same as your normal static pressure from the PRV.
If you have a tank, and it's good, then the prv might have failed. They usually last a lot longer than a year, though.