Before anything: the safety part
Switch the disposal off at the breaker, not just at the wall switch. A wall switch can be miswired, and someone can flip it while you are working.
Never put your hand into the chamber. Not to feel for the obstruction, not to pull something out, not with the power off. Use tongs, pliers or the wrench — there is nothing in there worth the risk, and the grinding components have edges even when stationary.
That handled, everything below is safe to work through.
Silence: no power reaching the motor
If flipping the switch produces nothing at all — no hum, no click — the motor is not being energised. Work through these in order, because they run cheapest and likeliest first.
The reset button. There is a small square button, usually red, on the underside of the unit. Disposals have a built-in thermal overload that trips when the motor has been strained or run hot, and it cuts power until it is manually reset. Press it firmly until it clicks and stays in. This is the single most common cause of a completely dead disposal, and an enormous number of service calls end here.
The breaker. Check the panel. A tripped breaker may look almost identical to an untripped one — switch it fully off and back on rather than eyeballing it.
The switch or the outlet. Many disposals plug into an outlet under the sink controlled by the wall switch. Plug something else into it to establish whether the outlet is live. Some installations are on a GFCI outlet whose reset is elsewhere entirely — often on the countertop or in the garage.
If all three check out and it is still silent, the motor or the internal wiring has failed, and that is a replacement rather than a repair on most domestic units.
Worth knowing
If the reset button trips again shortly after you press it, do not keep resetting it. That is the overload doing its job — the motor is straining against something, or failing. Repeatedly resetting a tripping overload is how a motor gets cooked.
Humming: power is fine, something is jammed
A hum means the motor is receiving power and trying to turn against something it cannot move. Leave it humming for only a second or two — a stalled motor heats fast, and prolonged humming is what trips the overload or burns the winding.
The fix is designed in. Most disposals have a hex socket in the centre of the underside, and nearly all ship with a small L-shaped hex wrench that fits it (frequently taped to the unit or left in the cupboard by the installer; a standard 1/4″ hex key works too).
With the breaker off, insert the wrench and work it firmly back and forth. You are manually rotating the grinding plate to break whatever is wedged between it and the chamber wall. It usually frees with a distinct give.
Then look into the chamber with a torch and remove what caused it — with tongs. Restore power, press the reset button, run cold water, and switch it on.
What is usually wedged in there
Worth knowing, because it tends to be the same short list, and the cause predicts whether it will happen again.
Hard objects that fell in: bottle caps, teaspoons, jar rings, children's toys. These are one-off accidents.
Fibrous vegetable matter wound around the impeller — celery strings, onion skin, corn husk. This is not an accident; it is a category of food that disposals genuinely cannot process, and it will recur until the habit changes.
Bone fragments and fruit stones wedged between the plate and the wall.
Accumulated fat and debris that has set hard enough to seize the plate, which usually means the unit has been running with hot water and greasy waste. There is more on why in what should never go down a drain.
The other faults, briefly
It runs but does not grind — the impellers are worn or broken, or the plate is spinning without engaging. Usually the end of that unit's life.
It runs, grinds, but drains slowly — the problem is downstream of the disposal, in the trap or the branch line. The disposal is fine; you have a drain issue.
It leaks. From the top, the sink flange seal has failed and can be resealed. From the side, the dishwasher or drain connection. From the bottom of the housing, the internal seal has gone and water is getting into the motor — that one is a replacement.
It smells. Not a fault. Debris has built up under the splash guard, which is the part nobody cleans. Lift the flaps and scrub the underside.
Two habits that prevent most of this
Run cold water, not hot. This is counterintuitive and it matters. Cold keeps fats solid so they get broken up and carried out; hot melts them so they flow down the line and re-solidify further along, where they cause a worse problem you cannot see.
Let the water run for 15-20 seconds after the grinding stops. Most people switch off the moment the noise changes, which leaves the ground waste sitting in the trap right where it can settle. The extra few seconds is what actually carries it away.