We've received reports that emails aren't showing up or being re-downloaded in the Mail app on iPhone and iPad. Our engineering team is currently working to resolve this issue as soon as possible.
To rule out any other issues, make sure you're using IMAP settings and use an app specific password if needed. You can also remove and re-add the account to the Mail app.
As a workaround, you can download the Yahoo Mail app to access your email.