Multi-pipeline branch unexpected disabled-- how to enable?

[Mark Stosberg]

I'm using the latest version of Jenkins and related Pipeline plugins, and our system has gotten into a stuck state.

A key branch is now in a "disabled" state for an unknown reason. The the multi-pipeline is driven by a Jenkinsfile which hasn't changed in several months. 

Now that the the last build status for the branch is "disabled", the Github webhook no longer triggers build, and I see no place in the UI to declare the branch "enabled" again. I've tried restarting Jenkins and rescanning the repo.

Suggestions for surgical fixes as well as large hammers are welcome.

Thanks,

    Mark