I agree with @janbb. There are many ways to reject a rejection. Take @avaeve‘s example of nihilism: one might reject it on the grounds of being a realist who simply takes the nihilist to be mistaken about what values exist. The traditional theist and the classical moral philosopher are both people who reject nihilism in this way.
As @Earthgirl mentions, however, one might reject nihilism in favor of existentialism. The existentialist takes a more direct interest in nihilism. Rather than simply embracing what the nihilist rejects, the existentialist has a more nuanced view that accepts parts of the nihilist’s arguments while denying the nihilist’s conclusions.
It seems to me that any case of rejecting a rejection will have analogous possibilities: some will deny that the original rejection was at all warranted, others will deny that the original rejection is the proper response to whatever led to it. That no single word exists for two such different stances strikes me as not all that surprising.