Ubuntu 12.04 changed itself to xubuntu and stopped working

[Bret Busby]

Hello.

I have an Acer V3-772G laptop computer, which has an Intel i-7 4702MQ
Haskell CPU/graphics controller and an nVidia GEForce GT-750M graphics
thing.

I have on the system, Debian 7, Debian 6, Ubuntu 14.04, and, what was
Ubuntu 12.04, installed.

After two yars of trying to get the computer working, I have found
that, of those operating systems, only the two Ubuntu systems have the
necessary drivers.

To get the interrface that I wanted, I installed the mate desktop
environment, along with a number of other desktop environments, and
have been using the mate desktop environment as the preferred desktop
environment, on both of the Ubuntu installations.

In the last week, the Ubuntu 12.04 installation changed the boot
splashscreen, to a blue one, and, after the blue splashscreen, the
screens (I use an external monitor) go blank, and nothing else is
displayed auntil I do a crash/reboot, using the power switch (holding
it down until the system stops breathing).

I have found that the blue screen displays "xubuntu", and in searching
for that, I have found that xubuntu is an xfce version of Ubuntu.

I had not wittingly switched the system from being Ubuntu, to xubuntu.

In using the recovery mode booting, and getting to a root shell
prompt, I tried to use apt-get remove xfce, in the hope that that
would get rid of the switch from Ubuntu to xubuntu, and it reurned
"xfce not found".

How do I reverse this switch to xubuntu, without having to reinstall
Ubuntu, if it can be done?

Thank you in anticipation.

-- 
Bret Busby
Armadale
West Australia
..............

"So once you do know what the question actually is,
 you'll know what the answer means."
- Deep Thought,
 Chapter 28 of Book 1 of
 "The Hitchhiker's Guide to the Galaxy:
 A Trilogy In Four Parts",
 written by Douglas Adams,
 published by Pan Books, 1992

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