The most possible reason is that something is set wrong in the permalinks.
You may set them in Settings > Permalinks, but in case that doesn’t work, you’ll have to modify your .htaccess file.
The .htaccess file is in your WordPress root folder (not in the theme).
Log in to the site via FTP.
The dot before its name means that the file is hidden by default (To display it in FileZilla go to Server > Force showing hidden files).
Upload the .htaccess file to your PC. Open it with the editor and add these lines:
Save changes in the file and upload it back to the file system via FTP.
Check the site, the issue has to be fixed now.
You may read more information about permalinks on the WordPress Codex website.
DFD Support Team