https://github.com/Microsoft/vscode-go/wiki/Go-with-VS-Code-FAQ-and-Troubleshooting Q: Auto-completions stopped working. What do I do?
Run gocode close in a terminal and try again.
If it still doesnt work, run go get -u github.com/mdempsky/gocode to update the tool. If you use the go.toolsGopath setting, then set GOPATH to the value in that setting before running go get so that the tool is installed/updated in the right location
If it still doesnt work, run gocode -s -debug in a terminal and try again. Results from gocode will show up in the terminal. If you see expected results in the terminal, but not in VS Code, log an issue in the vscode-go repo, else log an issue in the gocode repo.