If you can still access to your yahoo account, but suspect to be compromised sign in, and change your password immediately. Also update your secret question & answer, and make sure your filters, and alternative e-mail addresses are untouched.
If you fail to log in, don’t panic. Try typing your password in a visible field, and make sure your keyboard locale doesn’t change it.
Failing that, If you have a secret question&answer established, try apassword reset, by going to the yahoo mail login screen, and selecting I can’t access my account => My password doesn’t work. By answering your secret question, you can initiate a password reset directly from here. Make sure to change these, once you’re able to log in.
If you find, that your secret question&answer has been changed, click “this isn’t my question”. This will offer you earlier version of your secret question –hopefully set by you.
If you don’t have had any secret question&answer set, you can contact yahoo customer service to report a stolen account, using the yahoo mail contact form.