Only way to shut down iMac is power button

When I try to either shut down or restart my iMac (Mavericks) it goes to a grey screen but will not shut down. I must use the power button.

I have tried all kinds of ways short of a complete, clean reinstall.

Any ideas?

More information needed. Which iMac do you have, year and model? And what version of OS X is it running? And what have you done to trouble shoot the problem thus far?

And what happens when you activate Recovery MNode and check out the hard drive?

That happened with my Mac a time or two. I had a crazy amount of apps or tabs open, which wouldn't let it shut down, unless I clicked the red dot (tab closer) first.