It sounds like you must not have the Bootstrap files referenced properly. Based on your previous questions, it sounds like you're using Blazor Server, so you won't see an index file in your compiled assemblies. Make sure your _Host.cshtml file (in your Pages folder of your Server project) is correctly pointing to your bootstrap files. In your head area, make sure this exists:
<link rel="stylesheet" href="css/bootstrap/bootstrap.min.css" />
Then make sure bootstrap.min.css exists in your project at wwwroot/css/bootstrap/bootstrap.min.css
You could also try referencing a Bootstrap CDN in your _Host.cshtml file: https://getbootstrap.com/docs/4.6/getting-started/introduction/