What should a man do about his wife's repeated refusal to have sexual intercourse, citing fatigue, and is this refusal considered sexual frigidity despite her denial of it?

A wife's refusal to share her husband's bed without a valid religious excuse is forbidden, indeed a grave sin. Persistently advise your wife gently, inform her of the حرمة (prohibition) of her action, and investigate the reasons for her refusal. It might be due to your harshness, or something she dislikes about you, or your shortcomings in fulfilling her rights during intimacy. If the situation persists after examining the reasons, consult medical professionals or seek advice from the relevant section of the website. A wife's refusal to share the bed without an excuse is considered nushuz (disobedience/rebellion).