。
Mine's in the shop getting fixed right now.
Literal
I [of-の] (one) [topic-は] now repair-middle is [explanatory-んだよ].
僕のは with noun ellipsis — 'mine' (referring to something understood from context, probably a car or device). 修理中 ('under repair') uses the suffix 中 meaning 'in the midst of' — common for activities: 勉強中, 食事中. んだよ adds explanatory emphasis.