。
She didn't reply to my letter at all.
Literal
She [topic-は] my letter [to-に] at-all reply [object-を] didn't-send.
全然 paired with a negative verb is the textbook 'not at all' construction — modern colloquial Japanese also pairs 全然 with positive predicates (全然大丈夫 = 'totally fine'), a usage prescriptivists once decried but is now thoroughly mainstream. よこす is a slightly old-fashioned, somewhat curt verb meaning 'to send (toward the speaker)' — it carries a faintly negative or expectational tone, often appearing in contexts where the speaker resents what was or wasn't sent. 返事をよこす rather than 返事をする emphasizes the directionality (the reply should have come to me) and adds an undertone of grievance.