The first step is to look at the URL that caused the 404 and verify the file exist in the expected folder. Your original screenshot shows you only have the site.css in the Content folder. If you used the MVC template to create the project then you should see bootstrap css files too.
The default MVC template has the following files in the script folder.
It is not clear what happened. Maybe the files were deleted or moved??? You can always create a new MVC project and compare the differences with your project. Or perhaps explain what you are trying to do.
If you moved the files then you must also update the BundleConfig.cs file to point to the new file locations.