I uninstalled grunt with following command.
npm uninstall -g grunt
Then I again installed grunt with following command.
npm ins
You have to install grunt in your project folder
create your package.json
$ npm init
install grunt for this project, this will be installed under node_modules/
. --save-dev will add this module to devDependency in your package.json
$ npm install grunt --save-dev
then create gruntfile.js and run
$ grunt