Is a wife forbidden to her husband if she refuses his intimacy on the grounds of fatigue, and does his warning to her of prohibition take effect?

Your statement to your wife, "If I do not sleep with you tonight, you will be forbidden to me," carries two possibilities:

1. A promise of prohibition: In this case, nothing is binding upon you.

2. An immediate execution of prohibition: If you intended divorce, then it is a divorce. If you intended ẓihār (a form of conditional divorce where a husband declares his wife to be like his mother or another unmarriageable relative), then it is ẓihār. If you intended an oath or did not intend anything specific, then an expiation for breaking an oath (kaffārat yamīn) is binding upon you. This applies if you swore concerning your own action.

However, if the intention was that you swore concerning her action (e.g., "If you do not allow me to sleep with you tonight"), and she knew about it but then did not allow you, due to ignorance (of the consequence), then nothing is binding upon you, based on what some Shafi‘i scholars have favored, because she did not cause you to break your oath intentionally.