HTTP
429 Too Many Requests
What it means
The server is asking you to slow down. You have sent more requests in a short time than it will accept, often because of a rate limit on an API or a login form.
Where you usually see it
APIs, password forms after several tries, and pages that reload in a tight loop. Some content delivery networks also use 429 when they think a client is automated.
What to try first
Wait and try again later. If you are running a script, add a delay and respect any Retry-After heading the server sent. If you are a person hitting refresh, stop doing that for a few minutes.
When to stop
Repeating the request faster will usually make the block last longer. This is not a sign that the site is gone.