Force Close - open the multi-tasking bar (double tap the 'home buttom' or use multi-gesturing and swipe up):
a. iOS 6.x - press & hold an app until all begin to 'jiggle'; find the offending app(s) and press the 'red circle w/ minus sign' - return to the home screen & try again.
b. iOS 7.x - find the offending app - then press and drag to the top of the screen - return to the home screen & try again.