Why Does My iPhone Keep Restarting?
There are a couple easy things to do that usually resolve this issue. Before getting started though, backup your iPhone via iTunes on your Mac or via iCloud (go to Settings > iCloud). Once you have a backup, launch the Settings app on your iPhone and go to Settings > General > Software Update. Download and install the update. Once that's done, open the App Store app and tap on the Updates tab. Then tap on Update All to update each of the apps on your device.
These steps will most likely solve the crashing issue. If the problem persists, you may want to erase your device completely (Settings > General > Reset) and then restore from the backup you made earlier.