Looking at nothing but blue skies (or black skies at night)? Chances are you need to look down.
In the screenshot below, the camera pitch gauge (left of the screen) indicates +45 degrees:
How do you get out of this situation? A couple of things you can do:
1) Use a gesture to cause the camera to look down: in Flying mode (i.e. when you're not in Enhanced mode), place two fingers on the middle of the screen and move them slowly upwards. In Enhanced mode (which also is the mode you're in when you're at a pin), you can use just one finger to accomplish the same thing.
2) Press the "get me outta here" button, aka Fly up: that's the button at the top right of the map (the map with the up arrow). Press once and the camera will ascend to its max elevation above sea level and be oriented pointing down towards the earth.
The other possibility is that no terrain is loading because your device is offline (no terrain was cached locally on the device). If you're offline, you'll see a notification message and the info bar at the top of the map area will turn red as shown here:
What's the solution? Get back online! Or navigate to a location which you've visited recently for which data is still likely to be cached.