The ‘profile can’t be found’ is when someone’s screenname has been changed. The profile would be under his/her original name, though you may not know what it is.
For example, The Goq’s profile (hope you don’t mind, Goq) would be under TheGoq, because that was his original one. If you try The Goq, you get ‘not found’.
I don’t know about no posts being listed. Perhaps it’s something along the same lines.
As to how you change your screen name, I wish I knew.