When deleting a Level a meaningful warning should tell you what is going to be deleted before you hit OK.
This way you can locate the objects and place them in a different Level. Then try again until all the objects hosted on the Level have been re-hosted to a different Level. Only then you can safely delete the Level.
At the moment, Revit 2018, you try to delete a Level and a warning tells you that a view is going to be deleted. You click OK and a whole bunch of things disappear without warning. There is no undo available for this action. Only then you get an idea of what was deleted. Too late.
Ideally, we should be able to select ALL objects hosted to a Level.
When Unloading a DWG you get a warning that this action cannot be undone even though you can reload it just as easily. But when deleting a Level which has serious repercussions no such warning and no Undo. Very NASTY tool.