Get out of "orange selection" mode All the objects in my scene are suddenly circled with a dark orange stroke.
I can't seem to modify, delete or even select some of them...

Just a beginner at Blender, but I couldn't get an answer anywhere I looked.
The reason you can't select other objects is because you are in Edit Mode.
The reason for the orange outlines is likely due to having entered edit mode (&#8633; Tab) while all objects were selected (A):



While in edit mode, you can edit the mesh of the Active object, but cannot move any other objects. To select a different active object for editing, you'll have to first toggle back to Object Mode (&#8633; Tab) before clicking an object with  RMB.

Also see the blender manual.