Yosemite - Mail not working
A couple of things to try:

Go to System Preferences > Internet Accounts > Turn off any email accounts, and try launching Mail again. 

Open Terminal.app, and paste or type:     ~/Library/Containers/com.apple.mail/Data/Library/

Find the Saved Application State folder, drag it to Trash, and try opening Mail. Finder will re-create the folder, and this should clear out anything that may have been corrupted when you updated OS X.