Suddenly getting a black level editor

I was using UE4 and it was working fine. I started the engine later and now all of my previous projects, and any new projects I create, whether blueprints or c++, have a black level editor. If I create a new level, it says navmesh needs to be rebuilt. If I click play, I get the following error messages:

  1. One or more blueprints have been modified without being recompiled. Do you want to compile them now?
    Doesn’t matter whether I click yes or no, I still get the following two error messages,
  2. Failed to create entry point. Try another location, or you may have to rebuild your level.
  3. Failed to create PlayerStart at desired starting location.

I don’ t understand why this suddenly started happening, and I haven’t managed to find any useful information on how to fix this. Any help?