Yahoo has implemented some new security protocols that you might not be aware of. If you sign into your account via web browser on your PC you can fix this.
- Click the gear icon at the top right of the screen next to your portrait / name.
- From the pop-down menu select Account Info, and then the Account Security option on the left.
- Turn on the Two Step Verification if it is not already enabled.
- Use the Manage app passwords option to generate a one-time use password.
- Enter that password into your mail app on your PC
- Repeat the last two steps if you want to sync your phone to the account as well.