Hi Daniel,
Welcome to our community! 
Select the Terrain in your Hierarchy, then press F zu set the focus on the terrain. It might be that the focus is on another game object.
If the issue persists, disable “Occlusion Culling” in the Scene Camera. Bear in mind, though, that this might have a significant negative impact on the performance of Unity. The more Unity has to render in the scene window, the slower Unity becomes.
http://answers.unity.com/answers/1667448/view.html
I hope this helped.
See also:
- Forum User Guides : How to mark a topic as solved