One of our project would not show the rotation origin, which looks like this,
while in the rotate command. The stuff it showed looked like:
When we aligned two objects, it would not show the customary lock too:
No clues yet.
However, when we tried to insert windows / doors, Revit would not show any preview, but would insert them anyway. But it would not show the inserted element! Yet, it shows the opening cut made by these inserted elements!
That's the clue. The current (active) workset is switched off!!!