After running npm uninstall -g @angular/cli which uninstalls the Angular CLI, you would need to run npm install -g @angular/cli to have the Angular CLI installed again and be able to use the commands starting with ng.

@WillHelpYou is right - when changing Node.js versions you typically need to also run npm install -g @angular/cli since when you change your Node.js version (especially using tools like nvm or manually reinstalling Node.js), you often lose access to globally installed packages like the Angular CLI.