This is pretty specific but I've hit it numerous times and got ultra-frustrated because it usually happens precisely when you don't feel like doing side quests to fix your editor.
Symptom
Autocomplete no longer works.
You try to lsp-workspace-restart but that's futile.
You try to pkill gopls but it's useless.
The usual suspects
go.mod
Do you have a go.mod file? You should have a go.mod file.
go mod init <package_name>LSP workspaces
Is your LSP workspace set correctly or does it end up somewhere in
/home/user or equivalent nonsense when it should actually point to the
root of your project?
Look for your buffer in lsp-describe-session. If it's in the wrong
workspace, use these commands to fix that:
Emacs Commands (spacemacs bindings)
lsp-workspace-folders-add , F a
lsp-workspace-folders-open
lsp-workspace-folders-remove , F r
lsp-workspace-folder-add with the root dir of my project usually helps.
PS: Where does LSP persist the session?
The list of LSP workspaces is stored in ~/.emacs.d/.lsp-session-v1.