At a guess - and it can only be a guess, since you've provided zero context - you've linked to the CSS files using the local file path on your computer. For example:
<link rel="stylesheet" href="C:\Users\No\This\Is\Wrong\DontDoThis.css">
When a user accesses your site using an
URL, that link won't load. Even if the user had precisely the same version of your CSS file in precisely the same path on their computer, all modern browsers block websites from loading files from the user's local computer.
Instead, you should be using a relative path to your files - eg:
<link rel="stylesheet" href="/css/YesDoItThisWay.css">
Alternatively, standard files such as Bootstrap can be loaded from a Content Delivery Network (CDN):
<link href="https://email@example.com/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">