Stop running water first. Every flush, every load of laundry and every shower adds to what is coming up, and the drain it is coming out of is the lowest opening in the house. Then work out whose line it is, because that decides who you call and who pays: in most New Jersey arrangements the lateral from the house to the main is yours, including the part under the road, and the main itself is the town's.
The first hour
Shut the water off at the fixtures, not at the street. Nobody flushes, nobody showers, the dishwasher and the washing machine stay off. A backup is the house looking for somewhere to put water it cannot push out, and until that stops the level keeps rising.
Stay out of it. Sewage is a biohazard, not dirty water: keep kids and pets upstairs and do not start mopping in sandals. If the water is anywhere near outlets, a furnace, a water heater or an electrical panel, kill the power to that area from a dry spot, and if you cannot reach the panel without standing in it, call the utility rather than wading over.
Photograph it before you clean anything. Wide shot of the room, close shot of the fixture it is coming from, and a shot of the water line on the wall. If this turns into an insurance claim or an argument with the town, the pictures taken at 9pm are worth more than anything written afterwards.
Is it your line or the town's?
This is the question that decides everything that follows, and there are two quick reads.
Ask a neighbor. If houses on either side of you are backing up at the same time, or the whole street is, that is the main and the main is the town's. Call the municipal sewer department or the utilities authority on your bill, day or night, and they will come out. Do not hire anybody until they have looked.
Run the two minute test. If it is only your house, go to the lowest fixture and run water somewhere else. If the basement floor drain or basement toilet gurgles or rises when you flush upstairs, the restriction is downstream of both, which means the main line out of the house. That is the same test in every drain in the house is slow, not just one, and it is the fastest way to tell a single blocked fixture from a lateral problem.
Snake or dig? The part nobody tells you at 9pm
A blockage gets snaked and the relief is immediate. That is a drain cleaner or a plumber, it is the right first call on your own lateral, and it is the cheap end of this.
A failed pipe does not stay cleared. The signature is repetition: snaked in the spring, back by autumn, snaked again. Roots find a cracked joint and grow back into it, a section bellies and holds water, or the pipe itself has given up, which in a New Jersey house built between the 1940s and the early 1970s often means Orangeburg. Clearing a failing pipe buys months, not years.
What settles the argument is a camera. A sewer camera inspection shows whether the line is blocked or broken, and where, and how deep. It is worth doing before anyone quotes you a replacement, and it is the difference between a repair at one bad joint and a full run.
Who does what
A drain cleaner clears the blockage. A licensed plumber handles the plumbing inside the house. We are the excavation side: when the camera shows a collapsed, bellied or root-destroyed lateral, we dig it up and replace it, coordinate the permit and the inspection, bore under a driveway rather than cut it where that is the better answer, and put the yard back. On an emergency we can be on site the same day, and we answer the phone at night, which is when most of these happen.
One thing worth knowing before the bill arrives: a standard New Jersey homeowners policy generally does not cover sewer backup unless you carry a water or sewer backup endorsement. Check your declarations page rather than assuming, and do it before you agree to anything, because the answer changes how you want the work scoped.
If it is a septic system, not a sewer
Backing up on septic is a different problem with the same symptom. A full tank, a failed pump or a saturated field all present as slow drains and then a backup, and the first move is the same: stop adding water. If the alarm is sounding, the septic alarm has its own article. If you are not sure which you have, septic or sewer, and how to tell settles it in a couple of minutes.