So what happened/what did you do before it disappeared? The more information you can give us, the better chance we have of fixing it.
Please also tell us whether the phone is rooted, since that opens more possibilities.
If you just mean it's not on the homescreen, is it in the app drawer? If so it's possible that you just removed the shortcut (apologies if this is a stupid suggestion, but all we know of you or your problem is the 18 words you have written, so I have to consider all possibilities!).
Otherwise, just as a guess, go into Settings > Apps > All and look for the camera app there. If it's not there at all it's somehow been uninstalled. If it is, check that it's not been disabled.