wasn'tCasual explanatory form of じゃなかった, used to explain or emphasize that something was not the case. In the example, 気分じゃなかったんだ means “I wasn't in the mood.”
JA: 昨日は忙しくて、映画を見る気分じゃなかったんだ。
EN: I was busy yesterday, so I wasn't in the mood to watch a movie.