This is an ongoing issue - I’ve found closing Visual Studio and re-generating project files fixes it, something I have to re-remember every time I stop using UE4 for a period of time.
EDIT:
Thought I would add that you can include the UnrealVS tool (UnrealVS Extension | Unreal Engine Documentation) to Visual Studio and set the ‘Build Startup Project’ to show in the toolbar. Using this compiles your code, updates the editor and generates project files for you, which should fix the issue most of the time (otherwise restarting Visual Studio afterwards will).
Note that your classes need to actually compile for it to generate the needed project files.