。
Since mother was sick, I couldn't go there.
Literal
Mother [subject-が] sick was [because-ので] I [topic-は] there [to-へ] couldn't-go.
病気だったので shows the past-tense + ので reason pattern: noun + だった (past copula) + ので supplies a softer 'because' than から would. へ marks direction ('toward there'), distinct from に ('to there'), though they're often interchangeable for simple motion. 行けなかった is the negative past potential of 行く.