It is likely still on the phone, just not on the screen.
First check the other icons and make sure that they were not added together, making a folder of icons. Happens all the time. If you do find it was combined with another icon in a folder, just tap and hold the icon and then drag it back to location where you want it.
If it isn't combined to make a folder, then you need to either tap and hold the empty background of your screen to bring up the icons screen, or swipe up (from the bottom of the phone) to bring up the screen. It is different on different devices. Once the icon screen is open you can then tap, hold and drag the camera icon back to the location you want it.