My 13” MB Air is a 2017 version running MAC OS 12.6.3. I'm all Apple and have used their Mail app since the dark ages and never had this kind of problem. To clarify a couple of things:
1) I used the wrong verb… my Mail won’t load; it thinks it's open. It's icon is sitting in the dock with a little dot under it. There is no error message. There is just no response when I click the icon to Open it. I have quit it. I have force quit it. I have to turned off my MacBook and walked away from the thing. I know computers sometimes need a little quiet time to recoup themselves. But nothing changes.
2) Again, Mail on my iCloud account is working fine. Same for on my iPhone and iPad ( where I’m replying to you now).
3) Mail was working fine until Monday, mid-day, when it stopped loading new mail and sending. I was about ready to quit for the day anyway so I just left it alone. But I've been trying to figure it out since yesterday morning.
Thanks very much for anything you can suggest that might help.