Which question does the adverb clause answer?

Sarah really didn't feel like going out on Saturday night because she wasn't in a very good mood; she hadn't slept well the night before.

A: When?

B: Where?

C: How?

D: Why?

E: On (Under) what condition?

F: comparison/contrast

Short answer: You are answering why?

Why doesn't she want to go out? She doesn't want to go out because she's in a very bad mood. She knows that she won't be good company.

Remark

The sentence after the; (semicolon) does not begin with any kind of connective word. If you put a noun in place of she, you would see that the last part after the semicolon is a complete sentence.