The process of getting your big black box on the Playstation Network once more is a little more involved than just turning it on - you'll need to install an update to the system software, version 3.61, too. This new version will also prompt you to change your PSN password, which isn't entirely surprising considering the reason behind the update. Also obvious: this update is mandatory. After the update has been applied, you'll need to wait until the PSN is up and running in your country - which, by now, is probably everywhere - at least in Europe and the US.
"If using a PS3, your password can only be changed on your own PS3 (or a PS3 on which your PSN account was activated), as an added layer of security," the Playstation EU blog details, "If you have never downloaded any content using your account on the system, an email will be sent to the registered sign-in ID (email address) associated with your account when you first attempt to sign-in to PSN. This e-mail will contain a link that will enable you to change your password."
You can follow the phased restoration of the PSN here.