1. Power down the device and boot up while holding the volume up key. This will disable all jailbreak tweaks. If this fixes the problem, one of your tweaks is the culprit. If not, go to step 2.
2. Restore the phone. If you have wifi on stock iOS, the jailbreak must've done something it wasn't supposed to. If the problem persists, go to step 3.
3. It must be a hardware problem.
Cydia should've saved your blobs. Go to Cydia's home page and see which versions are listed.
Edit on that last point: You'll have to temporarily upgrade to iOS 6. If you have saved blobs for iOS 5, downgrading shouldn't be a problem. Just be careful if you at an unlocker - make sure to remove the basebnd upgrade before restoring.