I see that sometimes on a Note 3, and restarting the phone fixes it. if an app is really gone, what you mean is "reflash", not "reboot". First, everything you've done to the phone since you got it will disappear, so make sure EVERYTHING is backed up. Then, since it's a Samsung, be prepared to get bored to death. (They're easy, but the 10 minute first-start delay is very boring.) See [Samsung] How to flash Stock ROM via ODIN.