Updating iPad- Error message: "Could not start session with iPad"
Several things to try:


Soft reset your iPad by holding the power button and home button at the same time for a few seconds. The iPad will reboot. You won't lose your files so don't worry. When it completely boots up, try the sync again.
If the problem persists, then you may try to use iTunes to restore the OS. Be cautious when using this method because you may lose data. Maybe you should try syncing on another PC/Mac computer before you do this restore:


Connect your iPad to your Windows 7 (cancel sync if you know it won't go through), then click your iPad in the "device" list; Press the restore button and follow the prompts.