Per conversation between Gareth and Mike, it was determined the Landscape tool is in Game Mode by default.
Within the Landscape Tool, all UI elements are disabled by default. This has been observed affecting:
Press the "G" key (toggle Game Mode)
This issue is first observed occurring as of UE4/Main CL 3150232.
Landscape UI elements are not visible
All UI is consistently present at default settings
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-36843 in the post.