Go to the settings --> System Settings --> restore system defaults.
You'll then restart your PS3 system, and it'll boot up like the very first time you took it out of the box.
You'll then scan again for your wireless internet, and put in your internet password according to whether you have WPA or WPA-PK2 (TKIP) encryption.
Then, when it says it's connected to the internet, go to the internet test under network settings in the settings menu - click on test connection and get your new IP address.
Then you'll be able to sign into the PSN again.
This should work!!