Then simply keep holding the home button of the i Phone for 10 seconds: i Tunes should then notify you that it found a phone in recovery mode.
While we're not exactly sure as to what causes the problem, but there is an easy solution that does work.
Most users are still fond of i Tunes backup so they generally tend to restore their updated i Phone via i Tunes. When you find that apps don't appear even in i Tunes or if the sync doesn't really put all apps into the i Phone, you might want to head over to the “Purchases” section (under Quick Links) in i Tunes store.
This is what usually ends up in apps going “awol.”When you update your i Phone, try to restore it via i Cloud. What if you restored already (via i Tunes) and the apps are missing? You'll find the apps you've already downloaded via the App Store here.
Greenpois0n untethered jailbreak does not require SHSH blobs for i OS 4.2b3.
Connect your i Pod touch 4G, 3G, or 2G to your computer via USB cable and restore i OS 4.2.1.