HTTP Status Code for database is down

That's exactly what a 503 is.

503 means that the server was relying on connecting some other service, which did not respond in time.

Server Error 5xx

Checked up on Wikipedia and the listing there seems to imply that a 504 would be the one I'm thinking of. Quite possibly the link over is outdated.

So:

504 Gateway Timeout

The server was acting as a gateway or proxy and did not receive a timely request from the downstream server.

I would suspect that a 500 or 503 would be appropriate. 503 is generally used for overloaded or maintenance conditions, but I don't think it would be unreasonable to use it for your situation.