icloud backup - my ipad is hung with the dialogue box saying "this iPad hasn't been backed up in 4 weeks. Backup happen when this iPad is plugged in locked and connected to WiFi". Even after doing what is said, I am not able to do anything with the iPad -

My ipad is hung with the dialogue box saying "this iPad hasn't been backed up in 4 weeks. Backup happen when this iPad is plugged in, locked and connected to WiFi". Even after doing that, I am not able to do anything with the iPad - neither able to reboot nor able to get to the homescreen. The dialogue box doesn't go at all even after pressing OK a hundred times. I treid tacking the back up through itunes and through 3G conncetion but of no avail. Please help