ubuntu-16.04 window manage stopped working.

[Josef Wolf]

Hello,

today, I rebooted this box after an uptime of about three weeks. After this
reboot, the window manager stopped working.

- No dash
- no menu bar
- the only thung I can do is to right-click onto the desktop and start
  "Terminal" from the context menu.
- no window decorations
- windows are created in the upper left corner
- no window switching via <Alt>-<TAB>

I have never ever tweaked anything on my desktop. So I guess, it is
Unity/Compiz what I am using.

So how can I track down why it stopped working?

How can I select a different desktop environment to get the system into a
usable state?

Thanks in Advance!

-- 
Josef Wolf
jw at raven.inka.de