I have a normal 21 inch iMac on Lion and for some reason in the last week or so it has decided it wont shut down anymore. It begins the shut down normally, programs close and so on.

Then it goes to the normal white screen with the busy icon going clockwise again as normal.

then thats it. It stays like that for hours, until I decide to press the off button and shut it down.

I have no idea why this is happening can anyone help?