My phone's wifi was working fine and then it randomly just stopped. It finds wifi but when I press on it it always says "unable to connect" i've tried EVERYTHING and it won't connect to any networks at all. I restored, let the battery drain, airplane mode on airplane mode off, putting it in a freezer....

I searched it and it seems alot of people with the 4.2.1 were having issues so I downgraded to 3.1.3 but it's still not working. It's been like this for like 2 weeks.

Also, my phone is jailbroken so I can't take it back to apple and get it fixed.

1) Just because it's jailbroken doesn't mean you can't take it to Apple. You could just restore to a non-jailbroken firmware and bring it back if needed.
2) Does your IMEI/MAC address show up in the network settings pane?