The Datum marker has been deleted but the view level is still there... and for the life of me I cant reproduce it...! If anyone knows how to delete the datum's without deleting the actual level let me know...
Anyhow... if you do run into the issue where your datum's are no longer there but your levels are here's the solution.
Under the properties of the view set your Far Clipping to No Clip, then the datum's should show up (as long as your crop region is turned off).
Hope this makes scene!!