If you used VSCode on unity and autocomplete is gone:

Check the .NET version in Assembly-CSharp.csproj

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<TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>

Then go install the correct version of .NET framework developer pack

(4.7.2, for example)
https://dotnet.microsoft.com/download/dotnet-framework/net472

Restart VSCode and now the problem should be solved.

Also trying installing ONLY ONE OF the following VSCode plugins:

  • Unity Code Snippets
  • Unity Snippets Modified
You could also try to edit `Assembly-CSharp.csproj` in the project directory to match the version that you have already. 
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<TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>