Automatic updates & themes

Hi everybody!

I started to build a template using as a basis HTML5BLANK. Everything worked perfectly, but this morning I opened the website and found out that everything has changed. The theme that I built is now called “Twenty Eleven Child”, the css were replaced with those of the theme “Twenty Eleven” and everything is destroyed. I have a copy in HTML, but now I have to bring everything in WordPress-Code. Someone can tell me why and how can I prevent this from happening again?

Thank you ??