If your SAMSUNG M919 Galaxy S4 is stuck or frozen, You can perform a soft reset operation. Check out how to force restart Android 4.2 Jelly Bean. As a result your SAMSUNG M919 Galaxy S4 should reboot and start running again.
Pressand hold Power button and wait until the phone turns off, or Press Power button and wait for the menu in which select option turn off the phone.
Now press the Power button, and wait until the phone turns on.