** My garbage disposal is constantly stuck even though there's nothing in it. **
It could be some food material is still inside and jamming the unit. Fibrous food items like corn husks, celery, etc. can leave fibers behind which the disposer can't grind can end up getting tangled in the workings.
It is also possible that moisture has gotten into the unit's bearings making them tight or causing the rotor to seize/rust tight when not in use. It may also be possible that part of the disposal (like the rotating disk) is bent and jams against the housing at times.
In either of the latter cases, the unit may not be able to be repaired cost effectively depending on model of disposer it is.
If you have to look for a replacement, getting one which automatically reverses helps avoid occasional jams.