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You have to restore your iPhone from iTunes on your computer. You can restore it by putting it to DFU (Device Firmware Upgrade) Mode. DFU Mode is a mode which will allow you to restore your iPhone to working condition. Put your iPhone to DFU (Device Firmware Upgrade) Mode and restore it from iTunes. Follow the steps below to put your iPhone to DFU Mode;
Note : Put it to DFU Mode while connected to computer and also in DFU Mode your iPhone will remain blank. Timing is critical when you put it to DFU Mode, so keep trying until it goes to DFU Mode
Please let me know if that didn't work, so we can continue troubleshooting.