I know that a similar was asked on Intellisense not working golang test files, but since I am new to VS Code I didn't quite understand the solution. Also since I have a low reputation I couldn't comment for help.
What I know about my problem is that on Ctrl + Space VSCode used to show the list of functions associated with the Golang package, but suddenly one day it stopped working.
go.autocompleteUnimportedPackagesis enabled astruein your vscode settings.