I agree with some of the others, run a hose up the line from the tank. If its frozen, this will thaw it out.
Keep the hose nozzle up against the blockage.
If it's collapsed, you will get dirty water and sand coming back towards you.
And once you get it open, once a month dump one of those 3 packets of baking yeast down the drain and follow it with lots of hot water, works like a charm for ours.
I'll echo some others:
a) a little salt won't hurt it.
b) running water (hot or "cold") will melt it. Just feed a hose up there, and blast away
c) the tank level will be right back up to the pipe in a few weeks (or less). I'm guessing the leach field isn't, uh, leaching, as well as it should.
Problem solved.
Another 3 hours of snake action along with almost 400 gallons of the hottest water I could get out of the water heater and we got solids, toilet paper, and LARGE chunks of ice shaped like the inside of the pvc pipe.
The sewer pumping guy agreed that the tank was likely full when the mass melt began recently. With nowhere to go things backed up into the pipe.
Standing water freezes MUCH quicker than flowing.