Mail quits unexpectedly

I have an iMac model 11,2 with OSX El Capitan. Mail quits unexpectedly when I select iCloud in-coming message box.

It started by not logging in to my Mail account, asking for a password but not accepting the password. (Is there any place where to recover that password?). So as I couldn't use my account mailbox I would use the iCloud mailbox, but it quits on me. I've tried re-booting, some times it works , others it doesn't. I've been emptying trash and junk mailboxes, sometimes it does the trick, sometime it doesn't. I have other devises (iPad, iPhones, Macbook) where I can read my Mail, but not in my iMac which is my main working place.

Any advise will be very welcome.

DG