If you delete level you plan view will be erased too.
I think you deleted a Datum and your level are still exist.
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).
And here i will post you some ways to recover you datum.
- You can create new Datum Levels by copying existing Levels or Datums in Elevation or Sections. This is not recommended as it will not automatically create a Floor plan or Ceiling plan Views (as stated).
- While in an Elevation or Section, use the Level tool in the Basics side menu to create your new Levels.
- Make sure the "Make Plan View" box is checked above in the Options Bar IF you want a new Level with associated Plan views.
- Uncheck the box IF you only want a Datum level
- IF you have an existing Datum, and you now want a Plan view associated with it (or the View was deleted from the Project Browser).
- Simply go to the View pulldown > New > Floor Plan... (or Ceiling Plan...), Then associate these plans to the Datum you want.
- (Contrary to what Nelson said... the Basics Tab does not include a Ceiling tool - this is found on the Modelling Tab.)
- For clarification -
- reference Plan Views & Ceiling views (Blue tag)
- Datums are for referencing geometry but not viewing plans. (Black tag).
Hope this helps!
Eng. Iliev L. 



⁞|⁞ BIM Implementer, Project & BIM Manager, Coordinator, MEP Engineer
⁞|⁞ http://www.TechRoom.bg