If you are using windows platform, just check whether itune remembers iPad after downloading the os. It will restart the iPad in restore mode. If at this time iTunes doesn't recognize iPad, then windows has misplaced the drivers. Keep device driver open, if you see yellow exclamation mark against apple mobile device in recovery mode, right click the same, say update driver, chose manual. After two three screens you will see apple mobile device listed with few others. Select this and install the driver. Your problem will be solved.
I had the same issue.
Apple advised me to disable my virus scanner while restoring with my windows-pc.
If you have a Windows machine, try this, looks like it works.