Thank you for your reply. I did not try yet to reset All Settings. But, here is what I did.
I disabled my POP accounts and left only my Google account, and Mail successfully launched. I added back one of my POP accts and Mail launched successfully, again. However, enabling the other POP account caused Mail to crash again. I then read on an old post to disable email threading in the Settings/Mail, and this solved the problem. The second POP account did have alot of threaded emails, while the first did not have any.
I have reenabled email threading again, and it still seems to be okay. Maybe, in time, this problem will popup again. However, for now, Mail seems to be working.