Call and have the tank pumped is the first thing to do.

It may well have everything to do with not having th etank pumped.

If solids of ANY kind, including cellulose from TP, made it to the field from a lack of pumping the field could well have been wrecked.

The small openings in in the earth of the field that allow water to soak in may have become clogged rendering the field unable to dispose of liquid.

New fields are often rather expensive, and you may need to have enough land available for a new field.

Find the best plumber in the area that knows how to deal with septic and fields.

If you pump the tank at this point you will have to wait for it to fill again to find out if the field is working.

Pumping a septic tank is NOT to remove the liquid it contains (though that has to come out as part of the pumping) but to remove solids that have settled to the bottom of the tank and can damage a field.

My guess is that you have a frozen pipe like everyone else. The septic system would fail more gradually, and you would've seen problems before this.

I can only relate my story which is similar. My system is 25 years old and has been pumped every 3-4 years. About 3 years ago we had laundry water come up a few times through a drain in the basement. The problem went away after a few weeks. Then this summer it happened again. I knew this was a problem now. So I called my septic tank guy. He came out and looked. He said its possible the line between my 2 tanks is rusted/clogged. He probed the ground and said it 'feels" like cast iron pipe (I was impressed). So they dug it up. Sure enough the line was cast iron and it was clogged except for a 1" hole.It wasnt rusted through. Just clogged with gunk. He cut the cast iron out and replaced it with PVC. Works great now!

You need a septic company not a plumber.

There are numerous things that can be wrong and not pumping the tank for 13 years more likely than not won you a multiple thousand dollar leach field replacement.

When you don't pump your tank solids accumulate and get out of the tank into the leach field where they saturate the area and clog the waste water flow. That's why water backs up.

Not to worry, the big backhoes that septic companies have dig up frozen ground real easy.

"You need a septic company not a plumber. "

In most places septic companies ARE plumbers, though they may have the heavier equipment for septic work (many rent as needed for each job).

"In most places septic companies ARE plumbers" and in most places a different license is required.

Anywhere septic systems are in use there are usually septic companies that specialize in that work and that is who one would want. I would never call a plumber for a water softener although many sell and install them, very, very, very few plumbers understand how to size them and set them up correctly.