How do I back up my iCloud when it won't let me unlock?

A box popped up on my iPad saying that I hadn't backed up iCloud in 5 weeks despite the fact that I have wifi and I plug it in every night. I've had this box pop up before but usually I can click ok and go on to use my iPad. This time nothing happens when I click ok. The box stays there and I can't do anything else. It won't even let me unlock while this box is up. I tried pressing the off button but when it comes back on the box is still there. I tried plugging it in again and nothing. I've seen the suggestions for backing up iCloud but none address this specific problem. Please help!