Mail won't open anymore OS X El Capitan 10.11.5
OK I finally figured out how to fix it (yay).

I followed this http://c-command.com/spamsieve/help/how-can-i-rebuild-apple to delete the current index which then let me into Mail (finally) and get all my old emails back.  I am so happy to have it back. :-) Thanks for trying to help me!