At the right hand side of the Blender UI there is a collection manager that lists all objects, lights, cameras, etc in the scene with their respective collections. Every single object is disabled, how do I re-enable them all at once?
There may be a better way? But here goes..
In the (funnel) 'Filter' dropdown of the outliner, enable the 'Disable Selection' dart-icon, if you haven't already.
If there is a top-level collection, below 'Scene', containing all the others, Shift-click the dart icon next to it, to recursively set the 'Selectable' state of all the collections and objects beneath it.
If there is no top-level collection, you can temporarily or permanently create one.
ShiftA once or twice to collapse the Outliner view and make it easier to see what you're doing.
Right-click below 'Scene' to create and name a new top-level empty collection.
Select all the top-level collections and objects whose state you want to change, drag them over the new collection and Ctrl-drop them there. This will link everything to the new collection without disturbing the originals.
Shift-click the dart next to the new collection.
Once done, you can X delete the new collection of links, if you want to.
There is also the per-viewport 'View Object Types'setting in the 3D-View header to look out for, if objects are unselectable.