Without knowing what web server you are running, what operating system it is running on, what your web root is set to, where you uploaded the files or which user you used when uploading It is next to impossible to troubleshoot this issue.
That being said, here are a few tips:
1.) Check your web server's error log. This will be in a subdirectory in /var/log. The directory will vary depending on the OS and web server you're using. The log of the 404 error should show the requested URL.
2.) Check your web server configuration. This will be a subdirectory of /etc/ (on Ubuntu/Debian it will be apache2 or nginx, on CentOS/Fedora it will be httpd or nginx). In the sites-enabled subdirectory view the configuration file you see to find the web root. This will be specified as
DocumentRoot. The document root is what is shown for
yourdomain.com/ in a browser and all URLS start from that level when you're accessing your site.
3.) Check that the files you wish to access are located in the location noted in the error log, under the document root directory.
4.) Check that those files are owned by (or readable by) the user your web service is running under. This may be www-data, httpd, nobody, or something else depending on the software you are running.