Software Center Won't start For a few days now, my Ubuntu Software Center won't start. Ubuntu One also won't connect (not sure if that's related).
When I try launching Software Center from Dash, nothing happens. If I try running 'software-center' from terminal, I get this error message:
2012-03-19 10:26:43,316 - softwarecenter.ui.gtk3.em - INFO - EM's: 17 15 21
Traceback (most recent call last):
  File "/usr/bin/software-center", line 149, in <module>
    from softwarecenter.ui.gtk3.app import SoftwareCenterAppGtk3
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 82, in <module>
    from softwarecenter.ui.gtk3.panes.installedpane import InstalledPane
  File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/installedpane.py", line 43, in <module>
    from softwarecenter.backend.oneconfhandler import get_oneconf_handler
  File "/usr/share/software-center/softwarecenter/backend/oneconfhandler.py", line 25, in <module>
    from softwarecenter.backend.restfulclient import get_ubuntu_sso_backend
  File "/usr/share/software-center/softwarecenter/backend/restfulclient.py", line 36, in <module>
    from lazr.restfulclient.resource import ServiceRoot
ImportError: No module named restfulclient.resource

I'm running Ubuntu 11.10 AMD64
EDIT:
It started happening after I install all the dependencies needed to setup an environment to work on the Ubuntu Summit website. I'm assuming something I installed must not have played nice with the rest of my system.

Try running the following command to reinstall software center.

sudo apt-get install --reinstall software-center

If this does not work try:

sudo apt-get install python-simplejson


Please update your system in a terminal with sudo apt-get update && sudo apt-get upgrade.
If you get any faults there link those in your question, if this solves your issue then flag this to the moderators, if nothing happened drop a comment.

No clue if this will help you, but the same thing happened to me after messing with python-pip, python-setuptools and python-virtualenv.
$ sudo apt-get install --reinstall python-pkg-resources

fixed it for me.
Actually looking at your traceback says it all. Run this...
$ sudo apt-get install --reinstall python-lazr.restfulclient

This is a dependency of software-center. If you look up software-center online or in synaptic, you'll see it's required. But your traceback is saying it can't find that module.