Visual Studio struggles because Unity doesn’t use it. Unity does not care about VS. It’s just a code editor. It might as well be Notepad++ or Sublime. Unity doesn’t care. Because of this, Unity blows away the .sln and .csproj files anytime it feels like it and then VS gets all confusticated and confused, for me on a near-daily basis.
Thus it is necessary to keep this cribsheet handy so you can wipe Visual Studio’s chin whenever it drools.
This may help you with intellisense and possibly other Visual Studio integration problems:
Most often the fix is as simple as doing Assets → Open C# Project from Unity. Other times it requires more.
Other times it requires you also nuke the userprefs and .vsconfig and other crufty low-value high-hassle files that Visual Studio tends to slowly damage over time, then try the above trick.
Barring all that, move on to other ideas:
https://discussions.unity.com/t/778503
Also, try update the VSCode package inside of Unity: Window → Package Manager → Search for Visual Studio Code Editor → Press the Update button
Also, this: https://discussions.unity.com/t/805330/7