Plenty of these call-outs turn out to be a two-minute fix, so it is worth trying before you ring anyone.
Start here
If it hums but will not turn, the motor is fine and something is jammed. Switch off the power at the wall, use the small hex key that came with the unit to free the flywheel underneath, then press the reset button on the bottom. If it is silent, the reset alone often does it.
Old drains complicate it
Fitting a disposal to an original cast iron kitchen drain sometimes reveals the actual problem — a line already narrowed by decades of build-up that cannot take the extra load. If a new unit causes backups straight away, the drain is the issue rather than the disposal.
Salt reaches the underside
Housings corrode from the outside here as well as within. Water coming from the body of the unit rather than the connections means it has rusted through and cannot be repaired.
What it does not handle
Cooking oil and fat, which set further along the pipe and combine with sand already in the line.