The user has deactivated their account or has an incomplete profile. (Profiles without images or a main profile image cause a profile to become inactive.)
The user changed their username and can only be found using their new username.
The user blocked you or flagged your profile.
You blocked or flagged this users profile.
The user has completely deleted their profile from our system.
We have disabled or removed the users account from our system. (Least likely reason.)
And last of all....
It never existed. :-)