You need to check your log files, with that amount of information no one can help you.
Is this webmin login or is this all websites.
Please post your system, default means nothing as well.
what did you install ?
but as @stefan1959 says you have not supplied any system details you can click the icon (arrowed) which will copy those details to your clipboard and you can paste them into a post.
Please use a descriptive title about your specific problem.
You’ll need to post relevant log entries and more details. The client has no useful information about why the server is not responding.
So, check service status: systemctl status httpd (maybe, you’ve given us no information about your system, you might not be using Apache, or you might be using Ubuntu or Debian which calls Apache by a different name…you’ve made it remarkably hard to help you).
And, check the relevant log. Again, you gave us no information, so I can’t tell you what logs and how to find the relevant details.