Apple Mail in Yosemite has quit working

Apple Mail us suddenly quit working. Something is corrupted or damaged but I don't know what. When I open Mail, all options are greyed out and there is no more window. Nothing but the title bar.I can't check for mail,send or receive or access my account info. All greyed out. All my mail is still accessible on my iPhone app as well as a third party mail app called 'Airmail.' I am using that now. My Gmail comes through there as well and transfers to my cloud space just as before. All my settings are right. I have tried repairing disc permissions. I even downloaded Yosemite again. And I am getting the same results.