Have you tried resetting your iPhone by pressing and holding the power and home buttons until the Aple logo appears, then releasing both buttons and waiting for the iPhone to restart. If that doesn't work, you could try resetting all settings. To do that, go to Settings>General>Reset>Reset All Settings. This will not delete any data but will require you to re enter passwords on sites that normally remember you.
If this doesn't help, you could try updating your iPhone to iOS 8.0.2.
If that doesn't help, you may have to restore your iPhone and set it up from the latest backup. If that doesn't work, you may have to restore and set it up as a new phone. In any case it will update your iPhone to the latest iOS version.