[ubuntu] After logging in Unity does not show up in one user account

Ubuntu 14.04.4
I can log in to Guest account and everything works normally.
If I log in to the main user account, the password is accepted, but I only get a default background wall paper, and no Unity. I can log in using the tty shell, and everything seems fine, except of course there will be no unity.

I have searched for similar posts, and have found many, but I believe my case is unique in that logging into Guest if fine, with the problem arising only if I log into the main user account. As far as I am aware, this problem has not arisen as a consequence of updating, or setting up and customising user interface. Any help please.