Press Alt+L to switch on/off landscape.
If it doesn’t help then try to apply the retopologize tool on the landscape.
It’s in the Landscape–>Sculpt tab.
It’s a known issue if you play around with the landscape lod options.
Concerning splines which could be affected as well by this glitch (they become invisible): Try to move them a little bit and the corresponding segments should become visible again. I’ve had similar issues and this solved it.
Another possible solution for the invisible landscape problem could be to use the “Resample” option in Landscape->Manage->Resize Mode: Resample (with unchanged options):
https://answers.unrealengine.com/que…isappears.html