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