Hi,
Check the version of Visual Studio you have installed. There was an issue with 15.9.2 and Visual Studio Tools for Unity 3.9.02 which didn’t reload the project within Visual Studio after changes were made within Unity. For example, the addition of a new script in Unity would not be reflected within Visual Studio. If you opened the script from Unity it would open in Visual Studio but would not be associated with the project and as such had no reference to the other libraries which were required, such as UnityEngine.dll.
There’s a topic here with details here;
…but if you do have 15.9.2 then I would recommend upgrading to 15.9.3 which will also upgrade Visual Studio Tools for Unity to 3.9.03.