Not sure if has anything to do with it, just thought I would add just in case. 
Do you by any have more than one landscape in your level by accident? Have a look at the “Scene Outliner” panel with all of the objects that are in your level, there should only be one “Landscape” and one “Landscape…Gizmo”, if there are more than one (named Landscape0, Landscape1 etc) make sure to delete it and try again. the fact that you can walk around below the landscape is strange…
If that doesn’t help, are you able to move the camera around freely when you are under the landscape? If it isn’t colliding with the landscape it is something to do with the collision settings on your character, and/or, the landscape. Try placing a box on the landscape and see if you can walk right through it or not, that might help narrow it down a bit.