I have had septic systems for 20+yrs now.
it did not freeze did it? having a slight leak or running water to prevent water pipe freeze sometimes will cause ice to form in the sewer line. the steady trickle may start to freeze a drop at a time and will form an ice dam. this has happened to me.
if you can, open the septic access panel, the one they use to pump out the tank. paper which is the usual culprit will lodge at the point the pipe hits the baffle. clear that.
tank might have to be pumped to get rid of solids that do not bio-degrade. nothing goes in mine except paper and gray water from dishes and bathing. keep as much grease out as possible also because that can harden and cause damming.
if you decide to access septic tank: don't forget rubber gloves, and do not breathe the fumes which may be poisonous, not just unpleasant.
never call mr rooter, I call them mr ripper eg they will try to take more $$$ from your wallet than reputable plumbers. that comment from experience.