If the camera icon disappeared from the home screen of your iPhone (or iPad), the first thing to do is to check on other screens of your device. Search within the folders. If you do not find it, then it may be due to the fact that the use of the camera has been restricted.
Go to "Settings> General> Restrictions".
Enter your pass-code to access the Restrictions section.
Click on "Disable Restrictions" to enable all the applications listed, or selectively disable restriction for the camera app.