my mail won't open

well, the usual way to approach these things is that if the full thing won't work, you pare it back until something works, then build back up again, one item at a time to isolate where the problem lies.

The Very simplest things Mail can do is open with NO online email accounts at all.

so my suggestion is to go to

systerm preferences > internet accounts

I like to take a screenshot of that page, then write the account password on each email account.

Write down the account-names and passwords for your email account(s) and then delete them.

You may be able to keep your iCloud account, you can try that.

Then ty to launch Mail.