Your probably read that 68 648 009 dropbox accounts have been recently compromised. In the past years, companies like Linkedin, Adobe, Tumblr, Fling or MySpace were hacked and it is likely that your credentials were stolen by hackers if you had an account on one of these sites. It’s even possible that your credentials (Name, email and passwords) have been published on the web.
If you don’t remember if you were one of the victims of any of these breach, there is a very useful website that allows you to check if your email address can be found in one of these leaked list of credentials: haveibeenpwned.com.
You just need to enter your email address and press enter. Then, you’ll know.
But, as you don’t always use the same password on all the sites you use and as you change them quite often, you’re probably safe!