Try opening the terminal and typing:
(your root password)
I dont know if PackageKit will try to block Yum if it is running, I removed Packagespit first thing as it is junk. You may have to wait for it to stop running or turn it off before you can use Yum. If this works for you then I would suggest also running yum install yumex as this is a better front end for yum at this time.
BTW, you may have to run yum clean all before you can update from the terminal.