Cause
(if you don't care, skip ahead to the "Solution" section)
For some reason, you had { "key": ".", "command": "" }
added to your keybindings.json file (which you can check with the Preference: Open Keyboard Shortcuts (JSON)
command in the command palette). Either you added it by accident, or some bug happened, or an extension somehow added it at some point in time. From some issue tickets I've seen, this looks like a bug related to VS Code 1.78.
Multiple people with this issue have the christian-kohler.path-intellisense
extension installed, which recommends Windows users to add that keybinding in its readme (so I think this is a likely culprit (someone even raised an issue ticket to that extension's maintainer: Outdated readme advice breaks the editor on latest version #220)).
Related issue tickets in the VS Code repo:
For your reference / learning purposes, the above issue tickets were found by googling "site:github.com/microsoft/vscode/issues "command" "not found"
" and using Google Search's "Tools" menu to filter for results from the last month.
More notes: Another technique for debugging problems like this is to check if an extension is causing it, which you can do by doing an extension bisect. Other techniques for getting debugging info with keybindings can be found at https://github.com/microsoft/vscode/wiki/Keybinding-Issues.
Solution
Manually remove that keybinding from your keybindings.json file (open it using the Preference: Open Keyboard Shortcuts (JSON)
command in the command palette).
The fix that will prevent this from happening again will be made available in version 1.78.2 of VS Code.