If the dashboard for your Google+ Page is showing that you suddenly have zero followers, two things:
1.) You’re not alone.
2.) Google knows about the problem and is working on a fix.
I started noticing the bug yesterday: All Google+ Pages that I manage are showing that they have zero followers. It’s happening only on the main Dashboard page for all of my Pages (plus.google.com/dashboard), as well as the dashboard pages for each individual Page. The Pages themselves show the correct follower number, so users aren’t seeing the bug, only Page owners see it in the dashboard view.
A Google spokesperson confirmed that “this is only an issue within the Dashboard view — it’s not impacting how other people see your page. It’s a known issue that the team is working to fix.”