|What wireless modem are you using?|
Can you connect other devices to the modem?
If not, it's likely an issue with either the modems connections - make sure all cables are secure - or with the modem itself. If either are the issue you'll have to contact your ISP.
Are there any red or blinking lights on the modem?
Does the PS3 detect other modems?
If not, it might be a problem with the PS3, in which case you'll either have to contact Sony or find a way to use a wired (Ethernet) internet connection.
User Info Page Biography for full specs.