What does the error say or number?
iPad: Basic troubleshooting
Update and restore alert messages on iPhone, iPad, and iPod touch
iOS: Resolving update and restore alert messages
iPad: Unable to update or restore
Try and force your iPad into Recovery Mode
1. Disconnect the USB cable from the device, but leave the other end of the cable connected to your computer's USB port.
2. Turn off the device: Press and hold the Sleep/Wake button for a few seconds until the red slider appears, then slide the slider. Wait for the device to turn off.
3.While pressing and holding the Home button, reconnect the USB cable to the device. The device should turn on.
4. Continue holding the Home button until you see the "Connect to iTunes" screen. When this screen appears, release the Home button. iTunes should alert you that it has detected a device in recovery mode. Click OK, and then click Restore to restore the device.