where did my little orange banner go?

Do you mean the "Top Contributer" banner/badge? (Usually you get those for getting 10% "best answer" in a particular category.)

You may have answered more questions in your "best answer" category that weren't picked as "best answer" so your percentage went down.

Let's say you answered 100 questions in Psychology and 10 were chosen as "best answer" (10% best answers). You would then become a top contributer in that category. But as soon as you answered one more question (that wasn't chosen as a best answer) your percentage would become 10 out of 101 answers = 9.9% (which is less than the required 10% - even if only marginally.)

The Top Contributor Badge is awarded for high contributions in a single category. You can have up to three categories. The exact qualifications are kept secret but rumors indicate that it is a minimum of 10% best answers and about 40-50 individual answers in a single category in a month. There are no statistics available on answers or best answers in a category. You will have to keep track yourself if you ever want to try to figure out the system.

ℬ ℋ

I read that in order to keep it you have to stay active in your category and keep getting best answers.