When you’re jailbreaking your iPhone using redsn0w, if your iPhone screen is dead (blank or black screen) and the redsn0w is showing waiting for reboot, you need to put your iPhone back to normal mode (not DFU or recovery mode) and rejailbreak it again.
Put iPhone to normal mode – You should disconnect the iPhone from your computer first, and then hold both sleep or wake and home buttons together for few seconds and release both buttons when you see Apple logo on the iPhone screen. Once reboot done, you will see the familiar slide to unlock screen or slide for emergency calls screen on your iPhone.
Rejailbreak Apple iPhone – You should rejailbreak your iPhone again from the first step. Now, close the redsn0w if it’s still running on your computer and launch the redsn0w to jailbreak your iPhone.
Hope this will help you to solve your iPhone blank screen issue and you can always leave a comment or drop me a message here if you need any help.