Good morning...all of a sudden my email stopped working and the screen displayed nothing at all.

I tried to log out but it said to try again quit mail and choose log out from the apple menu. I went to quit mail but it was faded out and I could not click on it. I then went to the task bar at the bottom and tried to click on mail but nothing happened. I also turned off my computer and turned it back on again, email still not available. This just seems to be on my iMac. I am receiving email messages on my iPhone and iPad. Oh, Safari still works on my iMac!!

Help.....