iPhone lost and erased for no reason
If you bought the phone from some other person, and not from an official store, the seller might have stolen the iPhone and sold it to you and now the owner is looking for it (I doubt it since you had it for 2 years)

If you bought your phone from the official store, your iCloud ID might have been hacked. The solution to activating it would be to go to an Apple store, prove to them that you are the rightful owner and they will reset it for you. After that, if you have a backup, restore the phone from the backup. Also, change your iCloud ID password Immediately. If you cannot log in, it might have been hijacked. In that case, click forgot password and reset from email.