My Iphone 4s can't restore!!!

So the other day, I tried to replace my Iphone 4s screen and was successful. But later that day, my Iphone shut down randomly twice. The first time, I turned it back on and it worked but the second time, it wouldn't turn on at all...and I tried EVERYTHING!!! But I eventually got it to go to the iphone connect to itunes (restore) screen and I've been trying to restore for hours but it wouldn't let me restore due to ERROR 21! I've seriously tried everything (DFU mode, redsnow, modify hosts, replace battery). Can someone help me resolve this problem please?