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If you've recently updated to iOS 7, pull up the Control panel and look for the lock/rotate icon there. Try turning it on and off. This seems to have fixe most people rotation problems with the update.
Like the previous instructions, the icon in the Control Panel will either be a Mute or Lock switch, depending on your side switch settings in Settings > General.
If it's only that one app, it is possible that the app broke in the update and the developer's have not fixed it yet. Take a look at the develper's website to see if they mention any issues with iOS 7.