I suggest you check your account from the Yahoo Mail site to see if something happened to your account. This will help to make sure that there is no issues with the app itself.
If you can't see your emails and folders on the site either, there are 2 possible causes.
- Your email account is sync with a messaging app like Outlook, and by thinking that you are using the POP protocol, you cleaned up your inbox. Unfortunately, you were using the IMAP protocol and that also deleted everything on Yahoo's servers.
- Someone has accessed your account without your knowledge or your account was compromised. Take a look at your Yahoo Security Settings to review the recent activities. If you suspect something wrong, change your password immediately to secure your account.
Yahoo may help you to recover deleted emails in the last 7 days by submitting a restore request.