Hey Somsklit,
Thanks for the question. Let’s see if we can isolate the issue by restarting/resetting your device:
Turn your iOS device off and on (restart) and reset
http://support.apple.com/kb/HT1430
If the issue persists after a restart, let’s see if we can isolate the issue further by backing up and restoring your device as new:
iOS: How to back up your data and set up your device as a new device
http://support.apple.com/kb/HT4137
After erasing and setting up as a new device, your device is now in a factory configuration. Test to determine if erasing and setting up as a new device resolved the issue. Content on the device may have caused unexpected behavior.
If the issue is still present, you may want to contact Apple Support.
Thanks,
Matt M.