Try to Reset your device: Press and hold both the Sleep/Wake button and the Home button for at least ten seconds, until the Apple logo appears (ignore the Slide to Power Off option that shows up first).
If that doesn't help, try a Restore.
Like he said or you can try to reset the app double click the home button and hold down the app till the red circle with a white line shows up then, press the red circle, should reset the app.
I tried to reset my iPhone but nothing happen I'll try to restore it in DFU mode u mean that???
no dont mean that
-Double click the Home Button (Circle at the bottom of your phone with the square)
-It Should bring up the multitasking bar
-Then hold your finger down on any of the apps on that bar, and they should jiggle like your going to delete it
as well red circle with a white line it, will show up in the top right corner of every app, (on the bottom bar)
-and just clear all the apps on the bar by pressing that red circle.
it can fix alot of app related issues.