If you expand the References in the Solution Explorer on the right-hand-side of Visual Studio, if you scroll through, do you see several that mention Unity in that list?
Assuming you have Intellisense enabled within Visual Studio;
…the only reason I can think of as to why this would be happening would be because Visual Studio isn’t able to reference the required libraries. If they are shown within the References under Solution Explorer.
As mentioned before, there was a recent version of Visual Studio and the Visual Studio 2017 Tools for Unity which had an issue and were not correctly re-loading projects, this would mean that a script was opened on it’s own effectively, without the reference to the project and thus, without the reference to the Unity libraries under Resources - this would cause the behaviour you are describing.
Assuming you have no updates due to install for Visual Studio, Intellisense is enabled and those library references are displayed I cannot think of any other reason why this wouldn’t be working for you.
The only thing I could suggest really at this point would be to uninstall Unity and Visual Studio (thoroughly), the reinstall Unity, ensure Visual Studio 2017 Community Edition is enabled as a component to install and see if after a re-install things are any better.