from command line go to /var/www and type:
check the ownership of your files. The command above should show you the user/group for your index file. Is it the same as the files you copied over?
If you installed a web server from a repository, the user/group is likely www-data:www-data or nginx:nginx (depending on your web server). Ensure that the files you copied over have the proper ownership. You can change the ownership by:
chown <user>:<group> files
chown -R www-data:www-data /var/www/folder