The Safari app has disappeared from my iPhone?
Sounds like you have got restrictions turned on, but that doesn't necessarily mean that it is these restrictions that are causing you an issue, you could have turned these on long ago and forgotten what password was setup (for sure, someone has turned this on and a password is required as part of this process).

If the missing safar is due to restrictions, then there is simply no choice other than recalling the password, or possibly restoring the phone from an old backup on the chance that the backup pre-dates the turning on of restrictions.

But icons often go missing for other reasons due to a particularly mean bug.  You can reset your homescreen layout to restore missing icons when this happens, (warning, it will reset the location and grouping of all your icons when you do this) by going to  Settings > Reset > Reset Home Screen Layout.

One final note, you may have simply moved it somewhere daft into a folder or something by accident - can you find Safari in Spotlight search?