Mac desktop is just BLUE - nothing there

Hi!

 

I installed norton antivirus for Mac on my i-mac last night.  I did the online update and then I did a manual virus scan.  It found nothing.  So I turned off my computer and went to bed.  This morning, I got up and turned it on, and now all i have is a blue screen where my desktop is supposed to be.  No icons, no background picture, nothing except for a blue screen and my mouse cursor.  After 5 minutes, my screen saver pops up just like it is supposed to do, but then when I try to go back to my desktop screen, there is nothing but that blue screen again.  I have turned it off and on, that didn't help.  I also did an option command esc. (mac version of ctrl alt del) and it gives me the force quit screen, but there is nothing to force quit.

 

HELP!!!!!!!!   PLEASE!!!!!!!