Your Apple Developer Program membership has expired
So the other day I got an email from Apple saying that:
You are no longer a member of the Apple Developer Program and any apps you had on the App Store or Mac App Store are no longer available. Renew your membership to reinstate your apps for download, submit new apps, and get personal technical support.
I'll be honest and tell you that I initially thought that you can't upload any new apps if you don't have a Developer Account since that seemed totally logical. However, I didn't think that they would also take down any existing apps that you have in the App Store.
Ah, dear Apple, live and learn I guess...
— Nikola Brežnjak (@HitmanHR) February 13, 2016