As Dave stated, get most of the old fuel out. Adding additive under freezing doesnt do nothing. Most people miss this: it needs to be mixed above freezing and prior to gelling. Otherwise the additive just sticks to the sides of the tank, does nothing. I wouldnt use too much, probably no-lubrication or less. Diesel 911 will work, but probably sure its hardier on the machine, emergency use only. Gelled fuel and filters can come back to normal when warmed up, do you have that warm garage, but if the temp is the same or lower, change filters and fuel. Otherwise same situation will occur, good luck.