Why am I hearing a humming sound and nothing is working

The first thing to do is to make sure it is clean and free of debris as this can physically prevent it from working. The next thing would be to open it and check the grease level and the condition of the gear and the armature pinion and make sure they aren't worn or blocked by chunks of metal from wear. After time and use they can get hot, the grease can break down and that gear and pinion can be worn or jammed.

While in there it is important to see the condition of the bearing as a sticking one can also be the issue.

It is also important to check the field coil and armature because debris can get in there or buildup from the brushes wearing and prevent the armature from spinning all together. You can clean them with an electronics cleaner or non-flammable parts cleaner and compressed air. I see plenty of armatures and fields go bad at the shop I work at but they usually ground out and you wouldn't hear the humming. The brushes should also move freely in the holders and not be jammed.

Replace anything broken as needed and all of that should solve your problem.