App will not stop loading

I was playing a game (Pocket Frogs) on my iPad mini when an ad for another game app (Skyburger) popped up. I wanted to touch the 'x' to close the screen but accidentally touched on the screen and Skyburger started loading. The problem is that it won't stop loading and, strangely, Pocket Frogs is still visible underneath the loading page. I can't get back to Pocket Frogs or stop Skyburger from loading. It's stuck on a black & white "Loading" bar with Pocket Frogs visible (but not accessible) underneath the black background behind the loading bar.

How can I get this app to stop loading so that I can access Pocket Frogs again?

Thanks!