The server refused to serve the request.
WHAT IT MEANS
Access denied. Either permissions, a rule, or a block.
COMMON CAUSES
File or directory permissions wrong No index file in the directory and directory listing disabled A rule in .htaccess denying access Your IP address blocked Directory Privacy enabled without you realising A security plugin blocking the request
CHECKING PERMISSIONS
Directories should be 755, files 644. A directory at 700 produces a 403.
Check ownership as well; files owned by the wrong user produce this.
IF IT IS THE WHOLE SITE
Check .htaccess for a deny rule. Rename the file and retest.
Check whether the index file exists and is named correctly. index.php, index.html, index.htm — case matters.
IF IT IS ONLY YOU
Test from mobile data. If it works there, your address is blocked by the firewall. Open a ticket with your address.
IF IT IS ONE DIRECTORY
Directory listing may be disabled with no index file present. That is usually correct behaviour rather than a fault.
AFTER A MIGRATION
Permissions and ownership frequently transfer incorrectly. Reset them.