ipad won't reboot

My ipad is has a pop up screen up that says there isn't enough iCloud storage to back it up and it hasn't been backed up in 22 weeks, to plug it in and connect to WiFi. I have don't this and it will not go away. I have held the home and sleep button down to reboot it and it does nothing. I let the battery totally die and when I recharged it the message was gone, but it is back again and as before will not reboot. It is completely frozen. Can anyone help?