I had noticed yesterday that my Feedburner was showing zero subscribers. It had happened before so I was not mad or anything since it used to get fixed within minutes. When I woke up today to check the Feedburner stats, it was still showing zero subscribers. I wondered if it was only me who was facing the issue, but seems like everyone else is having the same problem as well.
I didn't notice it before but the guys at Feedburner have also made an official notice regarding the issue here. They say they are currently working to get it fixed. By the way, the issue is only with "showing stats", feeds will still get delivered to your subscribers and new users can still subscribe to your feed without any problem.
There's two things you can decide to do now : one, see the actual number of subscribers and wait for Feedburner to get it fixed or two, backup the email of all subscribers and move to a new service. I have not really thought of moving to a new service, because it's just a problem with the stats and more importantly, visitors can still subscribe to the feed, and the email gets delivered to them without any problem. So, I don't see why I should move to a new service. You might not feel the same though.
Here's how you can see the actual number of your Feedburner subscribers :
Login to Feedburner.
Click on your feed.
Go to Publicize tab.
Click on Email subscriptions and select Subscription Management.
Scroll a little down, and in the end you'll see the actual number of email subscribers.
Now, if you'd like to move to a new service, you'll need to backup all those subscriber's email address.
Click on View Subscriber Details (see the picture.)
Click on Export : CSV and an Excel file with all the subscriber's email address will be downloaded to your computer.
You can now move to a new service such as Mailchimp and have the CSV imported without loosing any of your subscribers.
Update : The feed counts are back up now. (on Sept 25, 2012)