Last week, some European PSN account holders were given a mandatory password reset following suspicious activity on the network. This week, the US PSN has apparently suffered a similar issue.
A post on the official PlayStation forums explains that "irregular activity" has been detected on some accounts, and the passwords of those accounts believed to be affected have been reset in order to "protect consumers and their account information."
Sony hasn't released any further details as to what the suspicious activity may have been, or whether or not it is connected to the recent launch of the PlayStation 4. It is likely more a precautionary measure than anything else, however; doubtless Sony is keen to avoid anything on the scale of the massive PSN hack that occurred back in 2011 -- an incident which still gives some people pause when considering whether or not to input their credit card details into Sony consoles, and for which Sony drew considerable criticism for staying quiet considerably longer than they should have. There is currently no evidence that accounts have been hacked or user information has been compromised.
In order to reset a PSN password, you need the email address used to log in to the account and the date of birth of the person who registered it, then go here to find out specific instructions depending on what system you're using to log in. If you lack either piece of information, you'll need to contact Sony directly either via phone (1-800-345-SONY) or live online chat through Sony's support site.