Installing the OS X Lion Update 10.7.4 (Client Combo)
reinstalls the Mac App Store for you which may help.
It's ok do this even though you are already running v10.7.4.
Restart your Mac after the combo is installed.
Try the App Store.
Thanks. I tried twice to reinstall the Lion Update 10.7.4 (Client Combo) per your instructions. All goes well until the final minute when it is "moving items into place", then it hangs up and does not finish the process. i cannot stop it or exit and have to do a hard shut down. Still no access to the App Store any more. I am stumped sinec it used to be fine.