Please note, some contacts might disappear from your contact list. That can happen because of 3 things:
2. User deactivated him/herself, or asked support to permanently delete the account;
3. You blocked or deleted the contact from your list.
We kindly inform you, that in the first 2 cases, our support team has no power to restore a profile and/or conversation or even give you the exact reason, because that would mean violating the other person’s right to his/her privacy.
In the third case, you can easily unblock the profile by going to the "manage blocked users' section in your settings.