We'd really need more information about your setup and what has been done before we'd really be able to help diagnose the issue(s).
You can start by checking your error logs and seeing what's showing up there. Since you're tagging this post with Apache, let's start there:
The following paths are based on standard paths used by Ubuntu's package manager (apt), so if you're not using Ubuntu, they may be a little different.
tail -50 /var/log/apache2/error.log
2). System Logs
tail -50 /var/log/systemlog
3). PHP Logs
tail -50 /var/log/php*.log
If you can copy those to a Pastbin or screenshot each output, I can see if there's an issue.
Also, if you can run
and take a screenshot of that as well.
My initial guess, baring any misconfiguration or configuration issues, Apache may have failed, didn't restart, and now the site is down, which could be remedied by restarting Apache. We'd still need to find the root issue though.