。
She's crazy about rock music.
Literal
[topic-は] rock [in-に] absorbed [explanatory-だ].
夢中 (literally 'in the middle of a dream') is a な-adjective/noun describing total absorption — being so engrossed in something that you're 'lost in it.' The pattern Xに夢中 marks X as the object of one's absorption with the particle に. ロック here is a clipped form of ロックンロール ('rock and roll'), referring to the music genre. The casual だ ending and short structure make this a punchy, conversational statement.