This is a common error when you change CHMOD to 777 or have incorrect syntax in .htaccess file.
Please login ftp and check CHMOD of all files and folders, folder permission is 755, file permission is 644, public_html folder permission is 750
After checking CHMOD, if you still cannot access your website, please remove all .htaccess files under public_html folder and its subfolders.
or
You May Exceeding resources,
It is possible your 500 error is caused by too many processes in the server queue.
