[Cal Metal]

Before tearing it down, examine the internal shifter rails/components to see if they are binding. It sounds as though the outside synchro hub is not moving at all on the splined internal hub. It could be seized but I would check the action of the rails and forks before tearing everything apart.

Also, if one of the dogs on the synchronizer is broken, that would prevent the shift from happening, by creating a cocking action on the sychro unit. A loose sychro unit will normally create a situation where the shifter pops out of gear as opposed to preventing shifting into that gear.