Are you getting any errors? Or seeing anything in the log? What if you close your browser session and open a new session? Is this for only one flow or all your flows? I would have expected to see a dot next to the flow name in the tab indicating the flow was changed, which I don't see (or is that added by SNUtils 🤔)?
It seems you have changed on of the OOTB Flows for Change Management which has a "read-only" protection policy and thus it shouldn't be possible. However, until Vancouver it was, resulting in a deactivated Flow which cannot be activated anymore. In Washington, it seems ServiceNow has fixed that annoying behavior, as you cannot deactivate any more read-only protected Flows. Coming back to your question: Basically, it is possible to activate such a read-only Flow again, but it requires some Update Set magic I cannot explain here.
