Over the past several years there have been high-profile cases of popular websites being hacked and users’ account details being compromised; in addition, some hacks have resulted in the attackers publishing or selling lists of this information on the internet. Some examples are the databases of MySpace, Adobe, tumblr and LinkedIn. The LinkedIn case is the most recent, with the breach occuring sometime in May this year, and with 160+ million emails and passwords being exposed.
So what can you do? There is a website (although please remember this is 3rd-party and we have no control over this), which you can find at https://haveibeenpwned.com which will allow you to input your email address. It will then run a check against it’s own collected information to see if your details have ever been exposed in the above way. If it finds any (or in any case if you’re in doubt), you should go to the relevant website and ensure your passwords are changed, along with any relevant security/recovery questions/numbers.
Note that this security issue occurs at the provider side, with the attack hitting the websites directly, and this is not anything under your control or to do with your own PC; this is a frustration we face, especially in the modern world where more and more of our information is given up to ever increasing accounts online. The best you can do is ensure your own security is in-place and then keep a keen-eye on your online account activity to monitor for anything suspicious.