Follow these steps to fix the blank Cydia icon after iPhone 4 iOS 5 jailbreak:
How to fix the White/Blank Cydia icon after iOS 5 jailbreak
1. disconnect your iPhone from the computer
2. run redsn0w on your computer
3. Click “Extras” and check “Just Boot” from the options provided
4. Redsn0w will ask you to connect your iPhone to the computer and turn it off.
5. Follow the on screen instructions from redsn0w and goto next screen by pressing “next”
6. Put your iPhone into DFU mode (yeah! same old one) – follow on screen steps to make your iPhone enter dfu mode
7. Once all goes fine, your iPhone will restart with pwnage icon (redsn0w files working on it) on the screen. Let it finish its tasks
8. Once the process completes and your iPhone boots up, you’ll see normal Cydia icon and Cydia launch will work properly without any errors or issue.