。
She was hooked on the video.
Literal
She [topic-は] video [in-に] was-engrossed.
熱中する ('to be engrossed, absorbed, hooked') literally means 'middle of heat' (熱 'heat' + 中 'middle, inside'), an apt Sino-Japanese metaphor for the burning focus of total absorption. The に here marks the focus or target of her engrossment, the same に that appears in 興味がある ('be interested in'), 夢中になる ('be crazy about'), 集中する ('concentrate on'). ビデオ originally meant videotapes specifically; in modern Japanese it has broadened to cover online videos and short clips, drifting closer to the English 'video' that gave it its name.