The apple iPhone is essentially a computer and like many other computers it needs a reboot from time to time. Now a reboot is different from just turning it off and on again, a special process has to be followed to tell the device to fully close down all applications and start them again from the scratch (this does not mean wiping off everything you have stored).
On a handset with a removable battery the process was quite simple, taking the back cover off and removing the battery whilst the phone is on and then replacing it was the most common way of rebooting, iPhones are a little different.
Step 1 :- Press and hold the home button (the circle in the bottom centre of the handset as you look at the screen) and the snooze/awake button on the top edge.
Step 2 :- Continue holding both buttons until the screen changes and the phone begins to restart (this can take 15-60 seconds).
Step 3 :- Let go of both buttons when you see a silver apple logo on the screen (the phone is rebooting).
It may take some time for the phone to reload/reboot from this stage, don’t be alarmed just let the phone restart naturally.