You can try the following steps to resolve CSS issues on your website:

Ensure you have a recent backup of your website files and database to prevent data loss during troubleshooting.

1. Clear your browser cache to ensure you’re viewing the latest version of your website.
2. Open your website in a browser, right-click, and select "Inspect" to open the developer tools. Check the Console tab for errors and the Network tab to verify that the CSS file is loading properly.
3. Verify the <link> tag in your HTML.
4. Debug your plugins and themes by deactivating plugins one by one and switching to a default theme to identify conflicts.
5. If you’ve switched to HTTPS, make sure all CSS file URLs are updated to use `https://`.
Let me know if further assistance is needed!