The problem can be caused by the application crashing when it is running. This then causes the iPod to freeze or lock, due to the effects on the device. You can restart your iPod, so as to solve the problem. You can also do a reboot of your device, so as to go back to normal function as before. You can go to,http://ipod.about.com/od/troubleshootingyouripod/qt/ipod-touch-reboot.htm, for details on how to reboot.