Yeah restart your router, to do that, all you have to do is turn off your modem and unplug your router then you wait 1 minute, once 1 minute passes, connect it back, restart your PS3 and try again.
If that doesn't work, when you are trying to go onto the online for MW2 and it says "connecting to matchmaking" (or whatever it says) just wait, give it 40 seconds, if it's still there by then just restart your router.