Welcome to iMore! Do you see this indicator at the top of your screen?
If so, swipe up from the bottom of your screen to open Control Center and tap the rotation lock button to turn it off. The indicator at the top should disappear and you should be able to rotate your screen again.
If, after all of that, you're still having an issue reset your device by holding the power button and home button down simultaneously until you see the Apple boot logo.