This is usually the result of a wrong files/folders permission setting or a wrong or not permitted directive in .htaccess file
Our cPanel servers are setup to only allow a maximum permission of 755 for files and folders on the server, setting permission to 777 may cause internal server error, to fix this problem use your FTP program like SmartFTP to recursively change permission back to an acceptable level. For .htaccess file, you can remove or comment the wrong directive line or rename the .htaccess to anything else like 1.htaccess for example