Many people have lost a lot of characters. Some were the Pet Avengers (like in my case), some were the minions (like in my friend's case). It has a pretty easy fix. Send the support team a message, and you'll get back your characters as soon as they can give it to you.

I thought it was the only one that had lost characters, I was relieved now that I saw it is "normal". Thanks for the tip, I'll contact the support right now.