Gizmodo reports that some hackers are claiming to have compromised 7 million Dropbox accounts. Dropbox says the login information being passed around are actually from other services for which the users used the same usernames and passwords.
Dropbox says most of the passwords were already expired, and it has expired rest just to be safe. If you were affected, you should be asked to reset your password the next time you log in. Do that.
If you use two-factor authentication, you should be fine, though you might want to change your password anyway from an abundance of caution.