Why is my mobile switch off?

Do you have a lot of apps open ?after it reboots does it work or is it constantly in a state of rebooting? If you have to many apps open and running in the background there could be an issue with lack of memory to run the app you want to open so the phone shuts down . If you do want to close or terminate an App, then press and hold the Home key on your phone. After a few seconds there will be a list of running apps .Then you can swipe the Apps off the screen to close them. Hope this helps