A plate that does not turn still heats — and that is the good news
Microwaves inside a metal box do not spread out evenly. They reflect off the walls and interfere with themselves, and the result is a standing-wave pattern: fixed bands where the field is strongest and fixed bands where it is nearly zero, roughly half a wavelength apart. At 2.45 GHz the wavelength is about 12 cm, so the hot and cold bands sit some 6 cm apart and they never move, because neither the walls nor the frequency moves. The turntable is the cheap way out of that: it does not stir the microwaves, it carries the food through the pattern so that every part of the dish spends time in a hot band.
Which explains the symptom that confuses people most: with the plate stuck, the oven still heats. It heats badly and unevenly — the middle of the dish cooks, the rim stays cold, or one side of a bowl boils while the other is tepid — but it heats. And that is useful information, because the faults that genuinely stop a microwave heating are on the high-voltage side, and they cost more than a new oven. A microwave that still warms food is a mechanical problem in a ten-pound part, not an electrical one.
The drive is a chain with four links: the motor, the shaft it turns, the plastic sleeve pushed onto that shaft, and the square hole in the bottom of the plate that sits on the sleeve. Any one of those slipping produces exactly the same symptom — the plate stands still while the oven hums — so the symptom alone cannot tell you which. The test that can is almost embarrassingly simple: lift the plate out and turn the shaft with your fingers. If it turns freely with a little resistance, the motor and the sleeve are both doing their job and the fault is above them, in the plate or the roller ring. If it will not turn, or spins with no resistance at all, the fault is below.
Three faults, three answers
Turning in jerks, or scraping is the roller ring: three small wheels in a triangle that carry the plate and keep it centred. They do not drive anything. When they get a burnt spill or a hardened ring of grease in their track, they stop rolling and start dragging, so the plate jerks, stalls and scrapes. Lift the plate, wash it, pick the debris out of the track, and put the ring back so all three wheels sit in their recesses. That is free and it is the most common fault by a wide margin.
Plate sits still, shaft turns freely is the sleeve or the square hole. The sleeve is a small plastic part pressed onto the flattened motor shaft, with a square peg on top that engages a square hole in the plate. Both ends wear: the round hole in the sleeve goes round on the shaft, or its square top gets its corners rounded off by the plate. Change the sleeve, which costs a few pounds and needs only the model number. Check the plate as well while it is out — if its square hole has worn into a circle, a new sleeve will slip just as the old one did, and you need a plate instead.
No movement and no hum, or a hum with a shaft that will not turn, is the motor or its drive. The motor is a slow synchronous unit, three to six turns a minute, mounted under the cavity floor. It is replaceable, and on a machine you otherwise like it is worth doing. Listen before you buy anything, though: a hum with no rotation is a stalled motor or a slipped coupling, while silence is a dead winding or a door switch that never closes.