I'm trying to kick out of Recovery mode with redsn0w and I do all the steps it says, all it does it put my iPhone 4 into a white screen and a gray flash and then a black screen help me get out of recovery mode pleaaaaaase!
Try tinyumbrella too.
If it still wont boot up then you might need a restore.
What model iphone you're using, what firmware and what are you trying to accomplish?