My iPad is stuck saying "Cannot Back Up"

I cannot get this window off my screen. Usually I can just press "Later" and it will go away, now "Later" and "Settings" do nothing. I cannot even shut down my iPad since the window is blocking me from sliding on the power off menu.

Has anyone else experienced this? I had just finished syncing my iPad with my computer when it happened. I have no clue what to do, right now my iPad is completely useless.