I've encountered this problem repeatedly a couple of times so here is a checklist I find useful:
1. Is your app run by 'node' or 'nodemon'? If it's nodemon, modify the package.json and use node instead.
2. Is your main app file named 'app.js'? If yes, change it to a different name 
3. Have updated your package.json file with modules that you recently locally installed (npm install
Feel free to push more items to it.