Are you talking about the app, or the web site?
The web site is at
https://activecaptain.garmin.com and is still functioning. I think there were some hiccups getting accounts moved over from Jeff. Mine worked fine so all I can say is you may have to re-register.
There's been a bit of
history with the apps. The most recent one does double duty as the interface to
Garmin hardware, but can still be used on its own to some extent. Likewise, some third-party apps which included AC data still
work, while others have dropped support. I'd start with the web site and be sure you have a working account before trying any apps.
But you actually raise a bigger question: Is AC still "Active?"
The jury is still out. The data around here is getting pretty stale, but some folks report seeing and making recent
reviews. I'd be thrilled to see it go back to the level of activity it had before Garmin acquired it, but I'm not optimistic.