In case you use any of the Yahoo services you should check your accounts and act immediately since the company confirmed the massive data breach from 2014 where 500 million users accounts were affected.
Yahoo gave an official statement where they mentioned that they will notify each affected user.
Each of the affected users accounts may include names, email addresses, telephone numbers, dates of birth, hashed password, security Q&A.
Nevertheless, in case you are affected or not you should immediately act and change your password and enable two-step authentication (instructions here) which gives you an additional level of security where you can even request to receive a text message or call to your phone that includes few numbers you have to enter prior to logging into your account.
Moreover, you can use Yahoo Account Key (instructions here) that eliminates the need to actually remember a password. Instead, you will just receive a notification on your mobile device and you just type yes and voila- you are sign in is done.
You should also update your security questions that you have stated once you created your e-mail address since those are potentially exposed now.
This is not the first and last massive data breach that happened over the past few years.
What you should do is to take a lesson from it and immediately install an anti-keylogging app that keep your mobile device or PC safe from any intruders.