I had not used the iPad for three weeks. When I opened it, the dialogue box appeared:
This iPad hasn't been backed up in 3 weeks. Backups happen when this iPad is plugged in, locked, and connected to Wi-Fi.
I checked one website that suggested I power down the iPad, but when I tried, the slider appeared but did not operate.
The next suggestion was to download a clean version of iTunes. When I started the process a dialogue box indicated that "Heather's iPad needed the latest version of iTunes." I have downloaded version 11.0.6 (I think that's right), but I still can't get past the dialogue box on the iPad.
Thanks Vinod! That worked and iPad is now being backed up.