A sump pump that will not start may have lost power, have a blocked float, a clogged discharge, or a failed motor. Because it operates around water, electrical safety comes first: do not touch wet plugs, cords, or the pump while connected. Begin with observations from a dry location.
Check power only from a dry location
See whether the circuit has tripped and whether the relevant ground-fault circuit interrupter (GFCI) indicates a fault, without entering water. Reset once only if the area is dry and no damage is visible; if it trips again, leave the circuit off.
Look at the float and pit water level
A float trapped low can prevent startup even as water rises. With power safely isolated, remove accessible obstructions. Do not bypass the float switch or hold electrical contacts manually.
Check whether the discharge is frozen or blocked
A blocked outlet can make the pump hum, trip protection, or fail to move water. Inspect the exterior end and visible piping; do not dismantle a pressurized line while the pump is operating.
Rising water with a dead pump needs a backup plan
If the level approaches the rim, limit water sources into the basement and use a backup pump only with a safe power arrangement intended for wet locations. If flooding or wet electrical equipment is a risk, call professional or emergency services as appropriate.