o.k, this is embarrassing:

I have installed Compiz Config Settings Manager and tried to fix it so that the transition time between changing tabs (using alt+tab) will be short. by accident I un-pressed V from something else, and it asked me about a conflict - I pressed the "x" button to close the window and as a result I stopped seeing the unity interface. That is - I can not see any buttons of the left side.

I went to the terminal (ctrl+alt+F1) and ran

ccsm

As a result I got the following error:

$ ccsm
/usr/lib/python2.7/site-packages/gtk-2.0/gtk/__init__.py:57: GtkWarning: could not open display
  warnings.warn(str(e), _gtk.Warning)
Traceback (most recent call last):
  File "/usr/bin/ccsm", line 93, in <module>
    import ccm
  File "/usr/lib/python2.7/site-packages/ccm/__init__.py", line 1, in <module>
    from ccm.Conflicts import *
  File "/usr/lib/python2.7/site-packages/ccm/Conflicts.py", line 26, in <module>
    from ccm.Constants import *
  File "/usr/lib/python2.7/site-packages/ccm/Constants.py", line 29, in <module>
    CurrentScreenNum = gtk.gdk.display_get_default().get_default_screen().get_number()
AttributeError: 'NoneType' object has no attribute 'get_default_screen'

What should I do next?

Thanks.