If the airbag goes off, the phone is probably totaled, call your insurance company.
If the airbag does not go off, restart the application. Apps are complex pieces of software and need to be restarted periodically to clean up temporary files.
If you recently got a new iPhone or tablet delete and reinstall the application. An application installed from a system restore may not have the most current libraries required for the application to operate. This is the most common reason that the application crashes as well. For detailed steps on clearing cache, checking permissions, and reporting bugs, see App Troubleshooting: Crashes, Errors & Cache Clearing.
For older iOS devices, double tap the home button to display all open apps and swipe the app up to close the application. For new iOS devices, from the Home Screen, swipe up from the bottom to show all open applications and swipe up mQ.
If crashes persist on an older device, check device compatibility requirements to confirm your phone or tablet is still supported.