The septic system is backing up.

Stop using water in the building completely: no flushing, no showers, no laundry, no dishwasher. Then call a septic contractor rather than a drain cleaning service. On a septic system, water you send down a drain has nowhere to go once the tank is full, and every load of laundry makes the inside of the house worse.

Why the instruction is stricter than on a sewer

A house on a municipal sewer has a blockage in one pipe. A house on septic has a system with a fixed capacity sitting in the ground, and when it stops accepting water there is no main under the street to take the overflow.

That gives you a specific and useful test. If the problem improves after a day of nobody using water, the system is overloaded rather than blocked. If it is just as bad, something is physically blocked or has failed.

What has usually gone wrong

The tank is full. The most common cause, and the most easily fixed. Tanks need pumping on a cycle measured in years, not decades, and the interval depends on tank size and how many people live there. If you do not know when it was last pumped, assume this one first.

The line from the house to the tank is blocked. Same causes as any sewer line: roots, wipes, grease, a collapsed section.

The outlet baffle or filter is blocked. Many tanks have an effluent filter that needs cleaning. A blocked one backs the tank up while the drain field is perfectly fine, and cleaning it is inexpensive.

The drain field has failed. The expensive one. The soil can no longer absorb what the tank sends it, usually from years of overloading, solids carried over from a tank that was never pumped, compaction from vehicles parked on it, or roots. The tells are soggy ground or unusually green grass over the field, a smell outside, and backups that return within weeks of a pumping.

The ground is saturated. After prolonged rain or snowmelt, a drain field in high groundwater cannot absorb anything. This one can resolve on its own as the ground dries.

A pump has failed, in a system that pumps effluent uphill to the field. There is often an alarm on these, on a panel near the tank or in the house, and people mute it and forget.

What to do while you wait

Stop all water use. Tell everyone in the building, twice.
Do not run the washing machine, which sends the largest single volume of water in the house.
Keep off the drain field, and keep vehicles off it.
If waste came up inside, close the area off and treat it as category three.
Find your records. When it was last pumped, tank size, and where the tank and field are. A contractor arriving without that spends the first hour probing the yard.
Do not add a septic treatment product hoping it clears. Nothing sold in a bottle empties a full tank or repairs a drain field.

What the contractor should do, in order

Locate and open the tank, and look at the level. A tank filled above the outlet says the problem is downstream, in the filter, the baffle, or the field. A tank at normal level with a backed up house says the problem is upstream, between the house and the tank.
Pump it. This is diagnostic as much as it is a fix, because the level tells them where to look next.
Clean or replace the effluent filter if there is one.
Check whether it refills immediately, which is the sign of groundwater entering the tank or of a field that is sending water back.
Camera the line from the house if the tank was not the problem.

Ask what they found, not just what they did. A pumping with no diagnosis is a service you will be buying again on a short cycle.

Insurance, and why this one is usually harder

Most homeowners policies exclude damage caused by a septic system backing up unless you carry the same water and sewer backup endorsement a city system needs, and they exclude the repair or replacement of the system itself as maintenance in nearly every case.

So the realistic picture is: the cleanup inside the house may be covered if you have the endorsement, and the tank, the field, and the line almost certainly are not. A drain field replacement is one of the larger unplanned costs a house can produce, and it is worth finding out now whether yours is near the end of its life rather than during a backup.

Making it not happen again

Pump on a schedule rather than when something goes wrong, and write the date somewhere you will find it.
Spread the water out. Four loads of laundry in one day is worse for a septic system than four loads across a week, because it is volume per hour that overwhelms a field.
Nothing but waste and toilet paper. No wipes of any description, no sanitary products, no cat litter, no grease, no paint, no solvents.
Keep the drain field clear of vehicles, sheds, trampolines, and anything with roots.
Divert roof water away from the field, since a field trying to absorb your downspouts has less capacity for the house.

The short version

Stop using water completely, then call a septic contractor rather than a drain service. Never open the tank. If it improves after a day of no water use it is overloaded; if it does not, something is blocked or has failed. Ask what they found, and expect the system repair itself to be outside your policy.