Gray screen w/ no dialog box

This problem developed yesterday. I'm sure everyone is familiar with how
Vista will gray out your screen and pop up a dialog box with "continue" or
"cancel" whenever you try to perform an administrative function. Now,
however, the screen will gray out but no dialog box will appear, and nothing
can be clicked on. Pressing enter does nothing. In order to get out I have
to CTRL ALT DEL and then just click cancel on that screen. Even when I get
out of the gray screen, I'm unable to perform whatever task it was that
required approval.

I would appreciate any suggestions