What does Http/1.1 Service Unavailable mean?
Http/1.1 Service Unavailable is a server error. If you notice a Http/1.1 Service Unavailable error while attempting to visit a website such as YouTube.com you may be directed to a white screen that says Http/1.1 Service Unavailable.
Many social networks such as Facebook, Google, and especially YouTube crash, or require maintenance which then may prompt a series of 500 server error screens and the Service Unavailable error in discussion.
This does not mean a website was hacked, though there is always a possibility. This is typically a server error where a website may lose functionality.