、。
I couldn't go to the concert because mother was sick.
Literal
Mother [subject-が] sick was [because-ので], I [topic-は] concert [at-に] couldn't-go.
A clean ~ので reason-and-result sentence: cause clause (mother was sick), main clause (couldn't go). Worth noticing the particle choice — に marks the destination 'concert' rather than へ. Both are natural for destinations with 行く; に slightly emphasizes the endpoint as a point one was supposed to arrive at, while へ would emphasize the trajectory toward it.