Are you saying that the main view of your document looks different to how it used to look? It shouldn't have changed, but there must be a reason behind it...
Could you tell me what version of OS X you are using, the specs of your Mac (is it a MacBook Pro, iMac, Mac Pro, etc? What CPU and graphics card has it got?) and if you go to 'File'->'Document Setup' what the 'Colour' tab says about your document?
Yenmei reacted to MattP in How to improve this foggy view?