Sony has informed PlanetSide 2 players on the EU forums for the game that an unauthorised third party has gained access to one of their systems and may have thus had access to players' personal data, i.e. email addresses and passwords. While the issue was apparently detected and dealt with quickly, preventative measures have been taken to ensure further problems don't arise.
If you try to log into PlanetSide 2 and can't, it may well be because your account has been suspended for security reasons, in which case you need to head to the forgotten password page to reset it. All players whose accounts may have been subject to unauthorised access have been sent emails requesting them to change their passwords.
New passwords must match the following criteria:
At least eight alphanumeric characters.
Use of upper and lower case.
The new password must not match previous passwords.
It's also suggested that players who use the same email address and password for other services change those passwords too, but it's really just good sense not to use the same password for multiple sites.
PlanetSide 2 is a free-to-play MMOFPS for PC that launched late last year, and re-imagines PlanetSide but on a bigger scale. The game was updated recently to improve the way players gain experience points, and it seems like it's pretty well supported. If you're put off trying it because of this security issue, remember that the security will no doubt be improved because of it.