The application that stops unexpectedly might be used by the system and that's why you can't use your device. Try to reboot your device. If the error still appears, the only solution remaining is to perform a factory reset of your device. To do this, turn OFF your phone, and press and hold the Menu button and Volume Up button until you see the Android logo on display. This means that you entered the Recovery menu. From there you need to select wipe data / factory reset. To be noticed that the key combination to enter this menu might be different for your device. In this matter, I suggest that you use Google to find the right combination if the one mentioned above isn't working.