Can you put your idevice into DFU (Device Firmware Update) Mode? Actually it is important to jailbreak, or downgrades your idevice correctly.please read this page if you need to know.
Please see the following steps carefully
- Make sure itunes is opened (use latest version)
- Need to do plug in your idevice to the computer via USB cable.
- Turn off your idevice
- Press & hold home button and power button at same time for exactly 10 seconds
- Then release the power button but don’t release home button.
- Keep holding home button
- In next few seconds itunes will recognize and pop up a massage which tells you that your idevice in recovery mode. Now release home button.
- If you have done correctly your idevice screen will go black (no logo no image.)