Try restarting your game, i.e remove it from your tray and launch again. For this purpose please do the following: double click the Home button - tap and hold TD:Reborn and tap the little red minus (if you have iOS 7 swipe the app upwards) - launch the game again.
If the problem persists, please try restarting your device.
If the problem persists, please try restarting your device.