Back in 2012, Dropbox was compromised and user data was extracted from the popular cloud storage site.
The details for the 68 million accounts including email addresses and encrypted passwords were made available on dark web marketplaces and a copy of the data was sent to security researcher Troy Hunt who confirmed that the breach was legitimate by testing two known accounts.
All users of Dropbox are advised to change their password immediately. Also if your Dropbox password is reused at other sites then you should change those passwords to unique passwords for each service.
For creating and managing passwords I highly recommend LastPass.
LastPass is available as a free utility but also offers a Premium version for US$12 a year that allows you to synchronise your securely encrypted password database across multiple devices so you can have your password vault securely stored on your computer, table and phone.
I also recommend visiting haveibeenpwned.com to check your email addresses against the list of known breaches to see what other services may have given up your online details.