。
I couldn't do my homework last night because of a long power outage.
Literal
Last-night [topic-は] long power-outage-of because-of I [topic-は] homework [subject-が] do-potential-not-[past-なかった].
Double topic marking with two はs in one clause — unusual but legal when the second narrows the first. ~のために ('because of X, due to X') is the formal reason phrase, more written than ~から/で. 宿題ができない uses the potential form of する (can-do) with 宿題 marked as the subject — 'homework was undoable.' Note the subject-marking が on what English would treat as the object.