My car's broken down, so it's in the repair shop right now.
Literal
I [of-の] car [topic-は] broken-is [because-んで], now repair-shop [in-に] entered-is [explanatory-んだ].
故障する ('to break down, to malfunction') is a noun+する verb. んで is a casual contraction of ので ('because'). 修理工場に入っている uses 入る ('to enter') with the ~ている resulting state — 'has entered and is now inside,' i.e. 'is in [the shop].' The んだ at the end frames the whole thing as explanatory.