You just upgraded/updated your firmware and it's on iOS 7.0.x. And suddenly, one of the two things happens: apps disappear from the homescreen or any app that you download does not show up on the homescreen at all.
Apple did not intend to play that prank but it happens for a lot of users. Fortunately, the fix for it is very very simple. In fact, it's so simple that most people find it out instantly.
1. Do a Soft Reset
Press the Sleep/Power and Home button simultaneously and hold them till the screen goes blank.
Now, let go of the buttons and then press the Sleep/Power button alone to reboot the iPhone.
This is a soft reset. Hopefully, this should fix the missing or disappearing home screen icons.
While this method fixes the missing or disappearing apps, sometimes, a soft reboot isn't enough. For instance, in a few cases, not only are downloaded apps missing, they don't even open even when you open them via App Store (post-installation). Now that's a graver issue. That calls for plan B.
2. Reboot Completely
Plan B is nothing huge. Instead of a soft reset, you turn off the iPhone completely. When you turn off the iPhone, you basically let it reset its bearings and clear the cache and stuff. So hold the Power button till the slider shows up, slide to power off and then restart the iPhone.
Missing icons after an upgrade could also be an issue with incomplete or stuttered update. If you find that your iPhone is freezing very often right in the middle of a process, throwing up errors or strange behavior besides missing icons, you might want to do a complete restore (after backing up the data via iCloud or iTunes). Make sure you don't do an OTA this time; connect to the computer and restore via iTunes.