Facebook has many methods to find dangerous activity on our accounts. When system will find something unusual, and in its opinion dangerous, account will be blocked automatically. Now Facebook introduced new way to unlock your account. All you have to do is.. taking selfie.
After uploading photo of your face, Facebook will analyze it. When photo will be considered as authentic after comparison with user's photos uploaded previously on website, then system will unlock your account. At the end photo will be deleted permanently from servers of Facebook.