Failed to load resource

Hi, my project is working fine until ytd. I am not sure why, when I “ionic serve” it, this error keeps on prompting and I realised that my build folder is only left with the following files:

I have already tried the below methods but all doesn’t work for me:

  1. Create a new project and put in my files.
  2. npm install
  3. npm install @ionic/app-scripts@latest --save-dev
  4. npm install -g ionic cordova

What I can do to install back my build folder?