mail won't load on my mac version 10.15.7

First make sure that you can log in to Yahoo! Mail using their webmail interface:

https://mail.yahoo.com/

Log in to that page using your email address and password. That will accomplish two objectives:

1. it confirms your login credentials (email address and password) are correct, and
2. it will allow you to receive the mail messages you have been missing for a few days.

A third objective could be that Yahoo! may require you to log in to that page every so often (perhaps every six months) to confirm it has not become dormant.

Let us know if that works or if you run into trouble. After that we can try adding Yahoo! back to the Mail app again.

In the meantime please find out your Mac's exact model designation and the macOS version it is using.

Excerpted from Find your Mac model name and serial number - Apple Support

From the Apple menu  in the corner of your screen, choose About This Mac. The window that opens includes the model name and serial number, ...

Yours might say something like macOS Catalina at the top, and iMac (Retina 5K, 27-inch, Late 2012) below it. What does yours say?

Please do not post its serial number. We don't need to know that either.

Ok thanks. I found this old Discussion with a solution that ought to work. Here are those instructions, copied and pasted for your convenience:

yahoo mail failed to authenticate - Apple Community

1. Delete your Yahoo! account from System Preferences > Internet Accounts.
2. Go to your Yahoo! account in your browser.
3. Click on your account on the top right.
4. Click Security.
5. Click Generate and manage app passwords.
6. Enter "Mail" when prompted for an App.
7. Copy the password Yahoo creates.
8. On your Mac, go to System Preferences > Internet Accounts.
9. Instead of clicking Yahoo!, click Add Other Account.
10. Click @ Mail account.
11. Enter your Yahoo! email address and paste the app password you just generated on the Yahoo! website.

This worked for me and I now have my Yahoo! emails actively coming into the Mail app on my Mac.

(I added a couple minor details for clarification)

Let me know if that works for you.

By the way getting to steps 4 and 5 might be different these days, but the ability to generate an "app password" should remain unchanged. If you run into trouble I'll try to get some screenshots with more detailed instructions.

Yahoo! also has some helpful instructions here: https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html#cont1

Review all these suggestions: If you can’t send or receive email on Mac - Apple Support and let us know how far you get.

If you still have trouble please write back. Don't tell us your email address — we don't need to know that.

🙂

You're welcome. Glad to help.