When Quick Look stopped working suddenly, I noticed how much I use it. Quick Look one of the indispensable advantages of OSX, whereby many files can be quickly previewed without opening any application. This feature has always been astoundingly fast. If you are used to waiting for Acrobat Reader to open, this can save you probably about an hour a week.
So, what to do if “cmd – y” only launches a grey rectangular window, and no preview of your file?
Apple Support Communities has the answer. Open the terminal and issue the following command to relaunch the Finder.
You could also use “cmd-alt-esc” (apple-option-escape in Apple language) to bring up the Force Quit Applications to achieve the same end. The answer is given for 10.6.8 but it also works on 10.8 Mountain Lion.