IPhone 4 4.2.1 stuck in recovery mode and won't restore after redsn0w jailbreak?

Hello, my iPhone 4 is stuck in DFU mode after I jailbroke it with redsn0w tethered and then ran greenpois0n rc6 to untether it. I cannot restore it, I cannot do anything. please help me! my iphone is on 4.2.1