Blown fuse 12 volt panel.. Since the furnace worked, and half the lights worked, and I assume the fridge and water heater or you'd have mentioned.
Blown fuse on house 12 volt panel.
Optional bad ground if all the failed lamps have a common ground. point.
Option 2: Bad connection.. Most likely on the 12 volt panel.. My 12 volt panel uses a type of connector that does not NORMALLY come loose but .. Well, this assumes it is fully connected
My 120 volt panel had screw so loose it was scary.. Some 12 volt panels also use screws.