Problem with Software Sources: Always performs an update instead of opening

Question

I cannot access Software Sources. Clicking it from the dash just doesn't show anything. Clicking it from the Ubuntu Software Center shows the Progress tab and remains busy briefly, but then when it's done, nothing happens. Trying Packages -> Repositories in Synaptic shows me the "Repositories changed" message and instructs me to click Reload, which I did three times and still got the same message.

Repeating any of this multiple times did not change the outcome. I suppose something got messed up because of a power failure when I was installing an app a while back.

Edit: When I run software-properties-gtk, I get the following output:

Traeback (most recent call last):
  File "/usr/bin/software-properties-gtk", line 32, in <module>
    import aptsources
  File "/usr/lib/python3/dist-packages/aptsources/__init__.py", line 1, in <module>
    import apt_pkg
ImportError: No module named 'apt_pkg'
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 64, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 4, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 20, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 21, in <module>
    import apt_pkg
ImportError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "/usr/bin/software-properties-gtk", line 32, in <module>
    import aptsources
  File "/usr/lib/python3/dist-packages/aptsources/__init__.py", line 1, in <module>
    import apt_pkg
ImportError: No module named 'apt_pkg'

Hmm, sounds like a module named apt_pkg is missing. I tried apt-get install --reinstall software-properties-gtk, but it didn't fix it.

Any help on what to do? Thanks.