Webpack not recognized as internal external command

I’ve successfully installed sage 9 on windows this week by following the docs recommendations. However, yesterday my site wouldn’t run after ‘yarn start’. Am receiving this error:

yarn run v1.22.4
$ webpack --hide-modules --watch --config resources/assets/build/webpack.config.js
‘webpack’ is not recognized as an internal or external command,
operable program or batch file.

Not sure why it has stopped working.