The sump pump has stopped and the pit is filling.

Check three things in this order: the breaker or GFCI outlet the pump is plugged into, whether the float is stuck, and whether the discharge pipe is blocked or frozen. Most sump pumps that appear to have died have not, and two of those three take under a minute to check.

While you check, get water out of the pit any way you can. A shop vacuum, a bucket, or a spare utility pump on a hose buys you time and stops the level reaching the floor.

1. Power

Sump pumps are usually on a GFCI outlet, and a GFCI in a damp basement trips on its own more often than people expect. Look for a tripped reset button on the outlet and press it. Then check the breaker panel for a tripped breaker.

Two other stupid and common causes worth ruling out in ten seconds: the plug has vibrated loose, or a piggyback float plug (the pump plugs into the back of the float switch plug) is not fully seated.

If the outlet is dead and the breaker is fine, the circuit is the problem, not the pump.

2. The float

This is the single most common failure and it is not a failure of the pump at all. The float is the switch. If it cannot rise, the pump never gets told to run.

Look for:

• The float pressed against the side of the pit, or against the discharge pipe.
• The pump sitting crooked, often because it has walked across the bottom of the pit from vibration.
• Gravel, silt, or a piece of debris jamming the float arm.
• A tethered float whose cord has wrapped around the pipe.

Free it, set the pump upright and centered in the pit, and lift the float by hand. If the motor kicks in when you lift it, the pump is fine and the float was the problem.

3. The discharge

If the motor runs but the level does not drop, water is not getting out.

• Frozen discharge line. In cold weather the exterior section freezes solid and the pump runs against a plug of ice. This is the classic January failure.
• A blocked or buried outlet outside, often under leaves, mulch, or snow.
• A failed check valve, so the column of water in the pipe drains back into the pit each cycle and the pump cycles endlessly without gaining.
• A blocked weep hole or an airlock, where the pump spins but moves nothing.

If the outside line is frozen and you cannot clear it, disconnect the discharge inside and run a temporary hose out of a window or door to somewhere the water will not run back toward the house.

4. The pump itself

If it has power, the float moves freely, the discharge is clear, and nothing happens when you lift the float, then it has failed. Motors burn out, impellers seize on debris, and most pumps last somewhere in the range of seven to ten years, less if the pit runs often.

At that point the job is to keep the pit down by other means until a replacement is in. A hardware store utility pump and a garden hose is a legitimate stopgap and much cheaper than a flooded basement.

Why the pit is filling at all

Worth a thought while you work, because it changes what to do next.

Heavy rain or snowmelt: normal. The pump exists for this. If it is running constantly and cannot keep up, the pump may be undersized or the water table is unusually high, and the outside work matters: downspouts, grading, and window wells.

No rain, and the pit is filling anyway: groundwater is high, or something is discharging into it. A dehumidifier line, a condensate line from an air conditioner or a high efficiency furnace, or a plumbing leak somewhere upstream can all feed a pit.

Water coming up around the pit or through the floor: hydrostatic pressure under the slab. That is a drainage problem, not a pump problem.

While the basement is wet

The same sequence as any other water event applies. Get people and pets out of standing water, lift what you can off the floor, get the water out, then dry what got wet.

Water that comes up through a sump pit has been through the ground. Treat it as dirty rather than clean: it is not sewage, but it is not drinking water either, and carpet and pad that sat in it are usually not worth saving. If the pit is tied into a system that also takes drain water, treat it as a sewage backup.

Insurance almost certainly needs an endorsement

This is the part to check before you spend anything. Most standard homeowners policies exclude water that overflows or backs up from a sump or a sump pump, in the same exclusion that covers sewer and drain backup. Cover for it is sold as a separate endorsement, usually with a limit well below your dwelling limit.

So there are three possible answers, and they are not the same:

• You carry sump and water backup coverage: the damage is likely covered up to that limit.
• You do not: the damage is likely not covered, even though the pump failing was sudden.
• The pump failed because of a power cut during a storm: check the wording. Some endorsements exclude failure caused by loss of power, some specifically include it, and some sit on whether you had a backup pump.

Report it either way and let them decide on the record. Reporting late is its own reason for denial.

What to change afterwards

• A backup pump. Either a battery backup or a water powered backup that runs off house pressure with no electricity at all. Almost every sump failure people describe is really a power failure during the storm that caused the water. If you are running a generator to keep it going, where it has to be.
• A high water alarm in the pit. They cost very little and they tell you the day the pump stops rather than the day the carpet gets wet.
• A check valve on the discharge if there is not one.
• Test it quarterly by pouring a bucket of water into the pit and watching the pump start, run, and stop. Not by lifting the float, which only tests the switch.
• Keep the pit clean. Silt and gravel in the bottom is what jams floats and seizes impellers.

The short version

Power, float, discharge, in that order. A stuck float is the most likely cause and it is free to fix. Get water out by hand while you diagnose. Check whether you actually carry sump backup coverage, and fit a battery backup and a pit alarm before the next storm.