Two different failures, one second apart, and they are fixed at opposite ends of the machine
The first thing everyone does is reach into the bowl and feel around. It is the wrong move twice over. The jam is not in the bowl: it sits between the turntable and the grinding ring, which is a horizontal gap you cannot reach from above — everything your fingers can touch up there was already empty. And what is down there is not a set of blades but two swinging lugs that are flung outward by centrifugal force, sitting in a chamber full of shattered bone and possibly glass. With the power off they will not move on their own, but your hand is still inside a container of broken things, and the next action in this job is going to rotate the turntable.
So use your ears instead, because the sound splits the whole problem. A hum means the motor has power and cannot move: that is a jam, and the answer is at the hex socket underneath. Total silence means the motor has no power at all: that is the overload protection, and the answer is the red button on the base — also underneath. Notice that neither answer is inside the bowl. The two are also frequently in sequence, because a jam draws a huge current, and a huge current trips the thermal protection within seconds. So the real order is press the reset first, then turn the hex, and if you do it the other way round you will turn a freed turntable and still hear nothing.
The physics explains the "it died overnight" story that follows these machines around. A universal motor at stall draws three to six times its rated current. The thermal cut-out is deliberately manual reset, not automatic — a self-resetting protection on a machine that a hand can reach into is a bad idea. So once it trips, the machine is silent and stays silent until a person pushes that button. It did not die. It is waiting.
Three faults, three answers
It hums and will not turn is a jam. Cut the power at the wall or unplug it — not the switch on the wall plate if the flex goes into a socket under the sink, pull the plug, because a switch is a switch and you are about to put a tool into the machine. Then find the hex socket in the centre of the base and turn it with a 6 mm key, both directions. Back and forth is what works: turning one way only drives the obstruction tighter into the gap. Once it frees, lift the obstruction out with pliers from above — you can reach it now that it has rotated to the top — and then run cold water and switch on briefly.
It is completely silent is the overload. Check the obvious first: the plug is in, the wall switch is on, and if there is a switch under the sink it is on. Then press the small red button on the base. It is a push-button that clicks and stays in when it has tripped, and pressing it is the entire repair most of the time. If the button feels solid and does not move, it has not tripped, and the fault is upstream of the machine. If it clicks and then pops straight back out when you switch on, the motor or its start winding is shorted, and that is a replacement rather than a repair.
It grinds perfectly but the water drains slowly is not the disposal at all. The waste pipe or the trap below the sink is partly blocked, and the giveaway is that the machine sounds entirely normal. On a double bowl, water rising in the other bowl is the same message. Do not touch the disposer: the part to open is the U-bend under the sink, which has a small cap or two slip nuts. Put a basin underneath, open it, and clear it out. Taking the machine off to fix a blocked trap is an hour of work and a new seal for nothing.