If you're using Apache, the best thing to do would be to check the error logs:
tail -20 /var/log/apache2/error.log
If on NGINX:
tail -20 /var/log/nginx/error.log
Also if on Apache, check for
.htaccess files. Back them up, delete them from the web root, and try to access again. Sometimes spaces, odd characters and other additions to
.htaccess files can cause an array of issues.
If you can post the output of the command above that applies to the web server you're using, we can take a look and see if there's anything specific in the logs that will help us to get you running.