The issue is likely caused by incorrect file permission settings.
Solution #
Using an FTP tool, connect to the server and set the permissions of the folders inside the WordPress installation directory to 755. (folders only)
wp-adminwp-contentwp-includes
Then, set the permissions of all files to 644. (files only)
If the following line exists in .htaccess:
RewriteRule .*\.(jpg|jpeg|gif|png|bmp)$ - [F,NC]comment it out to allow normal access.