We were performing a Server Maintenance earlier to prepare our Server for the imminent release of Version 4.0.
Unfortunately, there was a problem that caused many of you to lose their Ninjas. We are already working on a solution and everybody should have their Ninjas back by tomorrow. Furthermore, those Ninjas will then be active until our next Ninja Event (in ~38days).
We tried to cover everything when preparing our big update, but this one somehow slipped through.
Sorry for this inconvenience and thank you for your understanding,