Let's start with the bog standard initial response:
1) Reboot the iPad: Hold down the power and home buttons until the Apple logo appears (the power off slider may/may not appear and then the screen will go dark). The iPad will reboot. This usually takes a bit longer than a restart.
2) Shut down iTunes and reboot your computer. Then, open iTunes and plug in your iPad.
It may be that the newest version(s) of iTunes doesn't want to play well with your iOS 4.2.1. If you have your SHSH blobs saved for iOS 4.3.3 - it may be time to think about upgrading to that firmware version...
Regardless, if those two steps I outlined don't help, please come back and we'll see if we can try something else...
Marilyn