Okay, so you've jailbroken your iPhone, iPod Touch or iPad with RedSn0w for iOS 4.2.1, and now Cydia doesn't work. That's no problem–we will explain to you exactly why very shortly.
If you were here earlier, you may have read our posts on how do jailbreak iOS 4.2. You can check them out here:
You will notice that the jailbreak is tethered. We know that most jailbreakers already understand the concept of tethered vs untehtered, but there are many who are new to the idea and may not know exactly what tethered means.
When the jailbreak is tethered, you will HAVE to physically connect your iOS device to your computer and run RedSn0w to get the device to run in jailbreak mode.
If you simply turn your device on, your jailbreak apps will not work, and Cydia will be greyed out or will crash on launch.
1) Simply plug in your device and turn it off, and launch RedSn0w:
2) Choose the ipsw file of your current firmware:
3) Check off just Boot option as pictured below:
5) From there, you will be given instructions onscreen. Follow them exactly, and your device will boot.
You will have to do this each time you start your device, including the very first time when you first install the jailbreak and see that the Cydia icon is grey.
We can only hope that an untethered jailbreak comes soon. However, that may not be the case for a while, Chronic Dev-Team is planning their release, and it is also going to be tethered or semitethered.