In addition to Demo's excellent suggestions, is it possible you put in into a folder by accidentally moving it over another app or vice versa?
If all your Restrictions are turned off and your app is still missing:
1. Reset you home screen layout: Settings>General>Reset>Reset Home Screen Layout
- After resetting browse your home screens to see if it has reappeared.
2. If your application is still missing you will need to Restore the device.