I have a problem on my iPad. iPad is locked up with error message showing " This iPad hasn't been backed up for two weeks. Backups happen when this iPad is plugged in, locked and connected to WiFi" with an Okay button. The button does not clear error, cannot turn off iPad, cannot touch screen buttons. iPad is frozen. It does show that it is connected to our WiFi system. Any suggestions on how to clear and turn iPad off and restart?