Have you ever joined a security organization after it lost credibility within the business (for instance, after a major breach)? What did you do to build it back up?

Not directly.

But some years ago I was involved with the ISSA Generally Accepted Information Security Principles (GAISP)  project.
Since it was 100% volunteer-based and had no real leadership, it just sort of petered out.

I myself have not been involved with an organization that has had a major security problem. Helped an organization with a security (ransomware) problem. Most important was solving first. Provide the right information to the public and, after recovery, communicate openly and transparently about the attack, the steps taken and the approach to recovery. Of course within the standards of security and privacy. This restored confidence in the organization and normal services were quickly restored.

Not a security organization, but in the past I have joined a place which had lost its reputation due to other reasons. Its almost like building it back from scratch, like any startup would. Take small steps in building trust, and ensure you pay high level attention to customer retention. If you are truthful and open, people will gain back trust.