Is a jammed or leaking garbage disposal usually repairable, or does it need replacing?
A jam is almost always repairable. Most disposals have a manual reset and a way to free the flywheel from underneath, and that alone resolves the majority of jam calls without needing a new unit.
A leak at the sink flange, where the unit mounts to the sink, is usually a worn gasket rather than a failed motor, and replacing that seal is a straightforward fix distinct from the unit itself dying.
A unit that hums but does not turn, or does not respond at all, points toward a motor or electrical issue, and once a disposal reaches that point, especially an older one, replacement is usually the more sensible answer.
Checking the outlet and any reset switch under the counter is worth doing before assuming the worst, since a tripped breaker or a reset button that has not been pressed can look identical to a dead motor from where you are standing at the sink.