A fair number of disposal calls turn out to be something you can fix in two minutes, and we will tell you that on the phone rather than charge you for it.
Try this first
Humming but not turning means the motor is fine and something is jammed. Cut the power, free the flywheel with the hex key that came with the unit, then press the reset button underneath. Silent usually means the reset alone will do it.
When it is actually finished
Water leaking from the body rather than the connections means the housing has corroded through, and that is not repairable. Repeated tripping after a reset points to a failing motor.
Older sinks, older drains
Fitting a disposal to an old cast iron drain line in a pre-war kitchen sometimes reveals the real problem — a drain already narrowed by scale that cannot take the additional load. That is worth a camera look before assuming the new unit is faulty.
The grease it does not solve
A disposal grinds solids but does nothing about fats, which cool and coat the line. In older cast iron that coating collects on existing scale, which is how a working disposal ends up causing a blocked kitchen drain.